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COMPARATIVE  ANATOMY 
OF  VERTEBRATES 


KINGSLEY 


COMPARATIVE  ANATOMY 


OF 


VERTEBRATES 


BY 

J.  S.lKINGSLEY 

PROFESSOR  OF  BIOLOGY  IX  TUFTS  COLLEGE 


WITH  346  ILLUSTRATIONS 
LARGELY  FROM  ORIGINAL  SOURCES 


KC  ~ 

7  /  (^ 


PHILADELPHIA 

P,   BLAKISTON'S  SON  &  CO 

1012   WALNUT  STREET 
1912 


ZllQlo 


!   !    N 


Copyright,  1912,  by  P.  Blakiston's  Son  &  Co. 


THE. MAPLE • PRESS- YORK • PA 


PREFACE. 

Vertebrate  anatomy  is  everywhere  taught  by  the  laboratory  method. 
The  student  studies  and  dissects  representatives  of  several  classes,  thus 
gaining  an  autoptic  knowledge  of  the  various  organs  and  their  positions 
in  these  forms.  These  facts  do  not  constitute  a  science  until  they  are 
properly  compared  and  correlated  with  each  other  and  with  the  condi- 
tions in  other  animals.  It  is  the  purpose  of  the  author  to  present  a 
volume  of  moderate  size  which  may  serve  as  a  framework  around  which 
these  facts  can  be  grouped  so  that  their  bearings  may  be  readily  recog- 
nized and  a  broad  conception  of  vertebrate  structure  may  be  obtained. 

In  order  that  this  may  be  realized,  embryology  is  made  the  basis, 
the  various  structures  being  traced  from  the  undifferentiated  egg  into 
the  adult  condition.  This  renders  it  easy  to  compare  the  embryonic 
stages  of  the  higher  vertebrates  with  the  adults  of  the  lower  and  to 
recognize  the  resemblances  and  dijfferences  between  organs  in  the 
separate  classes.  There  has  been  no  attempt  to  describe  the  structure 
of  any  species  in  detail,  but  rather  to  outline  the  general  morphology 
of  all  vertebrates.  To  aid  in  the  discrimination  of  the  broader  features 
and  the  more  minor  details,  two  sizes  of  type  have  been  used,  the 
larger  for  matter  to  be  mastered  by  the  student,  the  smaller  for  details 
and  modifications  in  the  separate  classes  to  which  reference  may  need 
to  be  made. 

Considerable  space  has  been  given  to  the  skull,  as  there  is  no 
feature  of  vertebrate  anatomy  which  lends  itself  more  readily  to 
comparative  study  of  the  greatest  value  to  the  beginning  student, 
while  the  same  specimens  can  be  used  in  the  laboratory  year  after  year. 
The  skull  also  has  a  special  interest  since  nowhere  else  is  there  the  same 
chance  of  tracing  modifications  in  all  groups  since  the  first  appearance 
of  vertebrates  on  the  earth.  To  aid  in  this,  extinct  as  well  as  recent 
species  have  been  included. 

It  was  the  desire  of  the  author  to  adopt  the  nomenclature  of  the 
German  Anatomical  Society  ('BNA'),  but  this  was  often  found  im- 
practicable. The  BNA  was  based  solely  upon  human  anatomy  and 
it  fails  utterly  in  many  respects  when  the  attempt  is  made  to  transfer 
its  terms  to  other  groups.     The  single  example  of  '  transverse  process' 

v 


VI  PREFACE. 

is  sufficient  to  illustrate  this.  To  the  writer  another  objection  is  that 
the  BNA  strives  to  do  away  with  all  personal  names.  These,  it  would 
seem,  have  a  great  value  as  they  are  indications  of  the  history  of 
anatomical  discovery  and  memorials  of  the  great  anatomists  of  the 
past.  Dorsal  and  ventral  are  used  instead  of  the  anterior  and  pos- 
terior of  human  anatomy,  while  anterior  indicates  toward  the  head, 
posterior  toward  the  tail,  these  terms  being  readily  applied  to  all  ver- 
tebrates, man  only  excepted.  Cephalad  and  caudad,  adopted  by 
some,  lead  to  occasional  peculiar  phrases.  The  German  word 
'  anlage '  has  been  adopted  bodily,  and  seems  to  call  for  no  defense. 
It  implies  the  indifferent  embryonic  material  from  which  a  part  or  an 
organ  develops. 

The  illustrations  have  been  drawn  or  redrawn  expressly  for  this 
work.  Some  of  them  are  original,  some  based  on  figures  in  special 
papers.  Practically  none  have  ever  appeared  in  any  text-book  in  the 
English  language.  In  selecting  the  objects  to  be  figured  especial 
pains  has  been  taken  to  avoid  the  forms  usually  studied  in  our 
laboratories,  thus  relieving  the  student  of  the  temptation  of  copying 
the  figure,  instead  of  drawing  from  nature.  Especial  thanks  are  due 
to  Professor  C.  F.  W.  McClure,  who  allowed  me  to  draw  at  will  from 
the  splendid  collection  which  he  has  built  up  at  Princeton.  These 
figures  are  indicated  by  the  word  'Princeton'  followed  by  the  num- 
ber of  the  preparation  in  the  museum  of  the  University. 
Tufts  College,  Mass. 


CONTENTS 


Pagb 

Introduction i 

Introductory  embryology 6 

Histology i6 

Epithelial  tissues 17 

Nervous  tissues 19 

Muscular  tissues 20 

Connective  tissues 21 

Comparative  morphology  of  vertebrates 25 

Integument      25 

Skeleton 37 

Dermal  skeleton 39 

Endoskeleton 42 

Vertebral  column 44 

Ribs      53 

Sternum 56 

Epistemum 59 

Skull 59 

Skull  of  cyclostomes 75 

Skull  of  elasmobranchs 76 

Skull  of  teleostomes 77 

Skull  of  amphibia 82 

Skull  of  reptiles 87 

Skull  of  birds 95 

Skull  of  mammals 98 

Appendicular  skeleton 102 

Median  appendages 103 

Paired  appendages 103 

Shoulder  girdle 105 

Pelvic  girdle 109 

Free  appendages 114 

Coelom  (body  cavities)       120 

Muscular  system 124 

Parietal  muscles      125 

Visceral  muscles 132 

Dermal  muscles      134 

Diaphragm      135 

Electrical  organs 135 

Nervous  system 137 

Central  nervous  system 138 

Spinal  cord      138 

vii 


VIU  CONTENTS, 

Page 

Brain 140 

Brain  of  cyclostomes 152 

Brain  of  elasmobranchs 153 

Brain  of  teleostomes 153 

Brain  of  dipnoi 155 

Brain  of  amphibia 155 

Brain  of  reptiles 157 

Brain  of  birds 158 

Brain  of  mammals 158 

Peripheral  nervous  system 161 

Spinal  nerves 161 

Sympathetic  system 163 

Cranial  nerves 165 

Sensory  organs 177 

Nerve-end  apparatus 178 

Lateral  line  organs   " 179 

Auditory  organs 182 

Organs  of  taste 189 

Olfactory  organs 189 

Eyes 198 

Digestive  organs 205 

Oral  cavity 208 

Teeth 208 

Tongue 217 

Oral  glands 220 

Pharynx 222 

(Esophagus 222 

Stomach 223 

Intestine 227 

Liver 231 

Pancreas 234 

Respiratory  organs 235 

Gills  (branchiae)      236 

Pharyngeal  derivatives 245 

Swim  bladder 247 

Lungs  and  air  ducts 250 

Air  ducts      251 

Lungs 255 

Accessory  respiratory  structures 263 

Organs  of  circulation 264 

Blood  and  lymph 265 

Blood-vascular  system 266 

Embryonic  circulation 268 

Heart 269 

Arteries 273 

Veins 276 


CONTENTS.  IX 

Page 

Definitive  circulation 280 

Heart   .    .    .    .    , 281 

Aortic  arches 282 

Arteries 284 

Veins 289 

Foetal  circulation 293 

Circulation  in  the  separate  classes 294 

Lymphatic  system .  ^02 

Urogenital  system ^07 

Excretory  organs 308 

Reproductive  organs      319 

Reproductive  ducts 321 

Excretory  organs  of  the  separate  groups 326 

Reproductive  organs  of  the  separate  groups      331 

Copulatory  organs 342 

Hermaphroditism 346 

Foetal  envelopes 348 

Adrenal  organs 3^2 

Bibliography 354 

Definition  of  systematic  names 381 

Index 385 


INTRODUCTION. 

Any  animal  or  any  plant  may  be  studied  from  several  different 
points  of  view,  four  of  which  are  concerned  in  the  present  volume. 
We  may  study  its  structure,  ascertaining  the  parts  of  which  it  is  com- 
posed and  the  way  in  which  these  parts  are  related  to  each  other.  This 
is  the  field  of  Anatomy.  If  we  go  into  the  more  minute  structure,  for 
which  the  microscope  has  to  be  used,  we  are  entering  the  special 
anatomical  field  of  Histology.  When  two  or  more  different  animals 
are  compared  in  points  of  structure,  their  resemblances  and  differences 
being  traced,  the  study  is  called  Comparative  Anatomy,  and  it  is  only 
through  such  comparisons  that  we  are  able  to  arrive  at  the  true  meanings 
of  structure.  Then  it  is  of  interest  to  see  the  way  in  which  the  structure 
comes  into  existence  in  development  from  the  comparatively  simple  egg 
from  which  it  arises — the  province  of  Embryology  or  Ontogeny.  Anat- 
omy and  ontogeny  together  give  us  a  knowledge  of  the  form  and  how 
it  has  arisen  and  they  are  frequently  grouped  as  Morphology.  But  mor- 
phology merely  deals  with  the  parts  of  a  machine  and  these  are  usually 
studied  in  the  dead  organism;  fully  to  appreciate  the  mechanism  we 
should  know  how  the  parts  and  the  whole  perform  their  work,  the 
study  of  function  or  Physiology. 

In  view  of  the  foregoing  the  present  volume  is  to  be  regarded  as 
rather  a  comparative  morphology  of  vertebrates,  with  here  and  there 
hints  at  the  physiological  side.  Farther,  there  is  an  adaptation  of 
the  organism  to  the  conditions  in  which  it  has  to  live,  and  the  inter- 
actions of  this  environment  upon  the  animal  have  to  be  considered,  at 
least  to  a  slight  extent. 

Zoologists  divide  all  animals  into  two  great  groups,  the  Protozoa, 
in  which  the  organism  consists  of  a  single  cell,  and  the  Metazoa,  in 
which  the  body  is  composed  of  many  cells,  which  vary  according  to 
the  functions  they  have  to  perform.  Of  the  Metazoa  there  are  several 
divisions — ^Porifera  (sponges),  Coelenterata  (sea  anemoiies,  jelly  fish), 
Echinoderma  (starfish,  sea  urchins),  Platodes  (flatworms),  Rotifera, 
Ccelhelminthes  (ordinary  worms),  Mollusca,  Arthropoda  (crabs, 
insects),  and  Chordata. 


2  INTRODUCTION. 

The  Chordata  are  bilaterally  symmetrical  animals  with  metameric 
bodies,  which  agree  in  several  features  not  found  in  the  other  groups. 
These  are  (i)  a  central  nervous  system,  entirely  on  one  side  of  the  di- 
gestive tract;  (2)  the  presence  of  gill  slits  in  the  young  if  not  in  the 
adult;  (3)  an  unsegmented  axial  rod,  the  notochord,  between  the 
digestive  tract  and  the  nervous  system.  All  of  these  features  will  be 
described  later. 

There  are  three  or  four  divisions  of  Chordata,  the  uncertainty 
depending  upon  the  position  to  be  accorded  the  Enteropneusta.  These 
are  worm-like  animals,  occurring  in  the  sea  and  represented  on  our 
shores  by  Balanoglossus,  What  has  been  described  as  a  notochord  is 
a  pocket  from  the  digestive  tract,  lying  in  a  curious  proboscis  above 
the  mouth. 

The  next  division,  the  Tunicata,  includes  the  (marine)  ^sea-squirts.' 
They  were  long  regarded  as  molluscs,  but  the  discovery  that  the  young 
have  true  gill  slits,  a  nervous  system  on  one  side  of  the  alimentary 
canal,  and,  above  all,  a  notochord,  placed  them  in  the  present  associa- 
tion. Their  young  (larvae)  are  tadpole-like,  the  notochord  is  confined 
to  the  tail,  but  later  the  tadpole  features  are  lost  and  with  them  the 
tail  and  notochord,  and  the  adult  is  a  sac-like  animal  with  no  re- 
semblances to  its  former  state,  or  to  its  allies. 

The  third  division,  the  Leptocardii,  embraces  Amphioxus  and 
a  few  other  marine,  fish-like  animals.  They  were  long  classed  as  fishes, 
but  are  far  more  simple  than  any  true  fish.  The  body  is  markedly 
segmented,  the  gill  slits  are  very  numerous  and  the  excretory  organs 
open  separately  to  the  exterior  and  are  vermian  in  character.  Stomach, 
vertebrae  and  heart  are  lacking  and  the  brain  and  sense  organs  are 
very  rudimentary,  while  jaws  and  paired  appendages  are  absent. 

The  last  class,  the  Vertebrata,  are  most  nearly  related  to  the  Lepto- 
cardii, but  differ  in  many  important  respects-.  Thus  there  is  always 
a  skull  and  vertebral  column;  the  brain  is  larger  than  the  spinal  cord; 
there  are  always  nose,  eyes  and  ears;  a  heart  is  present  and  the  excre^ 
tory  organs  open  into  a  common  duct  on  either  side,  with  an  external 
opening  near  the  anus. 

Most  of  the  characteristics  of  a  vertebrate  may  be  seen  from'  the 
accompanying  diagram.  The  body  is  bilaterally  symmetrical,  with 
anterior  and  posterior  ends,  d6rsal  and  ventral  sides  well  differentiated. 
There  is  no  external  segmentation,  since  the  muscles  are  not  directly 
attached  to  the  skin,  but  a  metameric  arrangement  of  parts  is  notice- 


INTRODUCTION.  3 

able  in  muscles,  skeleton,  nerves,  blood-vessels,  and,  to  a  less  extent, 
in  the  excretory  organs.  There  is  no  cuticular  skeleton  but  the  outer 
layer  of  the  skin  may  be  cornified  or  the  deeper  layer  may  give  rise 
to  ossifications  (scales  of  fishes,  etc.). 

There  is  an  internal  axial  skeleton,  consisting  of  the  notochord, 
around  which  are  developed  rings  of  denser  material,  constituting  a 
backbone  or  vertebral  column,  while  in  front  a  skull  encloses  the  brain 
and  organs  of  special  sense,  and  gives  support  to  the  primitive  respira- 
tory organs  (gills) ,  which  are  always  connected  with  the  digestive  tract. 
Typically  there  are  two  kinds  of  appendages,  each  with  an  internal 
skeleton.  These  are  the  unpaired  or  median  fins,  dorsal  and  ventral, 
which  occur  only  in  the  Ichthyopsida,  and  the  paired  appendages, 
of  which  there  are  two  pairs,  anterior  and  posterior  in  position. 


Fig,  I. — Diagram  of  a  vertebrate,  a,  anus;  &,  brainy  c,  coelom;  da,  dorsal  aorta;  df^ 
dorsal  fin;  g,  gonad;  gd,  genital  duct;  h,  heart;  i,  intestine;  /,  liver;  m,  mouth;  n,  notochord; 
p,  pancreas;  pc,  pericardium;  pf,  pectoral  fin;  ph,  pharynx,  with  gill  clefts;  s,  stomach; 
sc,  spinal  cord;  sp,  spleen;  u,  ureter;  va,  ventral  aorta;  vc,  vertebral  column;  rf,  ventral  fin. 

The  central  nerv^ous  system  consists  of  brain  and  spinal  cord  which 
lie  dorsal  to  the  notochord,  and  are  usually  protected  by  arches  arising 
from  the  vertebrae  and  by  the  roof  of  the  skull.  Eyes  and  ears  are  the 
highest  of  the  sense  organs.  The  alimentary  canal  always  has  a 
liver  connected  with  it,  and  a  portion  of  the  canal  just  behind  the  mouth 
is  developed  into  a  pharynx,  from  which,  in  the  young  of  all,  gill  clefts 
extend  through  to  or  toward  the  exterior.  In  the  terrestrial  vertebrates 
these  gill  clefts  are  later  replaced  by  lungs  which  develop  from  the 
hinder  part  of  the  pharyngeal  region. 

The  blood,  which  always  contains  two  kinds  of  corpuscles,  flows 
through  a  closed  system  of  vessels.  A  heart,  ventral  to  the  digestive 
tract  and  lying  in  a  special  cavity,  the  pericardium,  is  always  present. 


4  INTRODUCTION. 

The  heart  consists  of  two  successive  chambers,  an  auricle  (atrium) 
and  a  ventricle,  and  in  forms  which  respire  by  means  of  gills,  contains 
only  venous  blood.  With  aerial  respiration  both  chambers  may  become 
divided  into  arterial  and  venous  halves.  A  dorsal  aorta,  lying  above 
the  alimentary  canal,  is  always  present. 

The  sexes  are  usually  separate.  The  reproductive  and  excretory 
systems  are  closely  related,  giving  rise  to  a  urogenital  system.  The 
excretory  ducts  usually  carry  ofif  the  reproductive  products  (eggs  and 
sperm).  The  urogenital  ducts  empty  near  the  anus.  Reproduction 
is  strictly  sexual;  parthenogenesis  and  reproduction  by  budding  do  not 
occur  and  alternation  of  generations  is  unknown.  The  viscera  are 
enclosed  in  a  large  body  cavity  (coelom)  which  in  the  adult  does  not 
extend  into  the  head.  Each  viscus  is  supported  by  a  fold  (mesentery) 
of  the  lining  membrane  of  the  cavity. 

For  details  of  the  classification  of  vertebrates  reference  must  be 
made  to  special  text-books  of  zoology,  but  as  some  of  the  larger  groups 
must  be  referred  to  frequently,  so  these  with  a  slight  definition  and  one 
or  two  examples  are  given  here. 

Series  I.  CYCLOSTOMATA. 

These  are  eel-like  in  form,  breathe  by  gills,  have  but  one  nostril, 
a  circular  mouth,  incapable  of  closing,  for  no  jaws  are  present. 
The  skeleton  is  poorly  developed  and  there  are  no  paired  appendages. 
— Lampreys  and  hagfishes. 

Series  II.  GNATHOSTOMATA. 

This  includes  all  other  vertebrates.  They  have  usually  two  pairs 
of  appendages,  true  jaws  and  a  well  developed  skeleton. 

Grade  I.  Ichthyopsida. 

Fish-like,  breathe,  at  least  while  young,  by  gills,  have  paired  ap- 
pendages, in  the  shape  of  legs  or  fins.  In  development  there  are  never 
formed  those  structures  to  be  described  later  as  amnion  and  allantois. 

Class  I.  Pisces. 

Fishes  respire  permanently  by  gills  developed  in  gill  slits  in  the 
sides  of  the  pharynx,  have  median  and  paired  fins  unless  the  latter  be 
lost  by  degeneration. 


INTRODUCTION. 


Sub-class  I.  Elasmohranchii. 


Fishes  with  cartilaginous^  skeleton,  mouth  usually  on  the  lower 
side  of  the  head,  the  gills  usually  opening  separately  on  the  neck,  and  the 
tail  with  the  upper  lobe  the  larger  (heterocercal) .  Sharks  and  skates. 
The  Holocephali  differ  in  having  the  gill  slits  covered  wdth  a  fold  of 
skin,  so  that  but  a  single  external  opening  appears. 

Sub-class  II.  Ganoidea. 

Intermediate  between  elasmobranchs  and  teleosts. — Garpike, 
sturgeon. ' 

Sub-class  III.  Teleostei. 

Fishes  w^ith  bony  skeleton,  mouth  with  true  jaws  at  the  tip  of  the 
snout,  gill  openings  concealed  by  an  operculum  or  gill-cover  supported 
by  bone.     Tail  with  upper  and  lower  lobes  equal. — All  common  fishes. 

Sub-class  IV.  Dipnoi. 

The  lung  fishes  are  tropical  forms  in  which  the  air  bladder  func- 
tions as  a  lung,  the  gill  openings  are  covered  with  an  operculum,  and 
the  tail  is  very  primitive  (diphycercal). 

Class  II.  Amphibia. 

Ichthyopsida  with  legs  replacing  the  paired  fins,  lungs  'present  and 
replacing  the  gills  in  the  adult,  nostrils  connecting  with  the  mouth. 

Sub-class  I.  Stegocephali. 
Extinct  amphibians  with  well  developed  tail. 

Sub-class  II.  Urodela. 

Amphibia  with  well  developed  tail,  gills  sometimes  retained  through 
life. — Salamanders,  Tritons,  newts,  efts. 

Sub-class  III.  Anura. 

Tailless  as  adults,  the  young  a  tadpole  with  external  gills. — Frogs 
and  toads. 

Sub- class  IV.  Gymnophiona. 

Blind,  burrowing,  legless  amphibians  occurring  in  the  tropics. — 
Caecilians. 


0  INTRODUCTION. 

Grade  II.  Amniota. 

•  Vertebrates  in  which  there  are  never  fins,  never  functional  gills, 
the  respiration  being  by  lungs.  In  development  the  embryo  becomes 
covered  by  an  embryonic  envelope  called  the  amnion,  while  a  second 
outgrowth  from  the  hinder  end  of  the  digestive  tract  is  concerned  in 
the  embryonic  nutrition  and  is  called  the  allantois. 

Class  I.  Sauropsida. 
Body,  at  least  in  part,  with  scales,  eggs  large. 

Sub-class  I,  Reptilia. 

Cold-blooded  vertebrates,  the  whole  body  covered  by  scales  or 
horny  plates.  The  living  forms  are  turtles,  lizards,  snakes  and  alli- 
gators (crocodiles)  and  a  New  Zealand  species  Sphenodon.  The 
fossil  forms  are  more  numerous  and  include  Theromorphs,  Plesiosaurs, 
Ichthyosaurs,  Dinosaurs,  and  Pterodactyls. 

Sub-class  II.  Aves. 
The  birds  are  recognized  by  their  warm  blood  and  their  feathers. 

Class  II.  Mammalia. 

The  mammals  are  as  sharply  marked  by  their  hair  as  are  the  birds 
by  their  feathers.  They  have  warm  blood;  except  the  monotremes 
they  bring  forth  living  young  which  are  nourished  by  milk  secreted 
by  glands  (mammae)  in  the  mother. 

There  are  a  few  other  terms  of  convenience  which  may  be  defined 
here  as  they  will  save  much  circumlocution.  The  term  Teleostomes  is 
applied  to  ganoids  and  teleosts,  from  the  fact  that  they  have  true  jaws. 
The  amphibia  and  the  amniotes  are  frequently  united  as  Tetrapoda, 
from  their  possessing  feet,  in  contrast  to  the  fishes  with  fins. 

The  geological  history  of  these  groups  is  important;  their  first 
appearance  and  their  geological  range  is  indicated  in  the  accompanying 
table  of  the  geological  periods. 

INTRODUCTORY  EMBRYOLOGY. 

The  structure  of  an  adult  vertebrate  can  be  fully  appreciated  and  the 
bearing  of  the  facts  recognized  only  by  a  knowledge  of  the  develop- 
ment of  the  parts  concerned.  It  would  often  appear,  for  example, 
that  certain  organs  in  different  groups  were  exact  equivalents  of  each 


INTRODUCTION. 


g  2? 
cr  c 
2.         2. 


Ostracoderms 

P  alaeospondy  lus 

Elasmobranchs 

Ganoids 

Teleosts 

Arthrodira 

Dipnoi 

Stegocephals 

Gymnophiona 

Urodela 

Anura 

Theromorphs 

Plesiosaurs 

Chelonia 

Ichthyosaurs 

Rhynchocephals 

Dinosaurs 

Squamata 

Crocodiles 

Pterodactyls 

Birds 

Monotremes 

Marsupials 

Edentata 

Insectivores 


Chiroptera 

Rodentia 

Ungulata 

Sirenia 

Cetacea 

Carnivores 

Primates 


Table  showing  the  geological  distribution  of  the  various  groups  of  vertebrates. 


8  '  INTRODUCTION. 

Other — duplicates  in  function  and  details  of  structure — while  a  knowl- 
edge of  their  development  may  show  that  they  have  had  entirely 
different  origins  and  different  histories,  and  hence  cannot  be  identical ; 
they  are  examples  of  what  the  evolutionist  calls  convergent  evolution. 
Such  cases  are  apt  to  lead  one  astray  as  to  the  relations  of  the  forms 
in  which  they  occur.  Farther,  the  development  affords  a  framework 
around  which  the  details  of  organization  may  be  arranged  in  a  logical 
manner,  thus  aiding  in  their  remembrance.  For'  these  reasons  the 
following  pages  are  based  on  embryology.  Not  only  are  the  histories 
of  the  separate  organs  traced  before  an  account  is  given  of  the  adult 
conditions,  but  this  introductory  chapter  gives  in  the  most  generalized 
form  the  earlier  stages  before  the  organs  are  outlined. 

The  enormously  complicated 
body  of  every  vertebrate  is  derived 
from  a  comparatively  simple  special- 
ized cell,  the  egg  or  ovum.  This 
ovum,  must  be  fertilized  by  a  still 
more  specialized  cell,  the  spermato- 
zoon, derived  from  the  male.  After 
Fig.  2.— Successive  stages  in  the  seg-  this  fertilization  the  egg  goes  through 

mentation   of    an   amphibian  egg.     1-7,  ^^Hprlv   hut   vprv    (rrflHiinl    c:prip«^ 

Results  of  the  corresponding  cleavage  ^^  oroeriy  Dut  very  graouai  serics 
planes.  of  changes  which  bring  it    contin- 

ually nearer  the  adult  condition.  The  phases  of  this  differ  with 
different  animals;  here  only  a  generalized  account  will  be  given,  which 
is  subject  to  modifications  in  the  several  groups,  for  an  account  of  which 
reference  must  be  had  to  embryological  text-books. 

The  Segmentation  of  the  Egg. — The  first  steps  of  the  process  are 
the  segmentation  or  cleavage  of  the  egg,  in  which  it  divides  again  and 
again,  until  the  single-celled  egg  is  converted  into  a  large  number  of  cells 
or  blastomeres  (fig.  2).  The  character  of  this  segmentation  is 
modified  accordingly  as  the  egg  is  large  or  small,  as  it  contains  varying 
amounts  of  nourishment — deutoplasm  or  food  yolk  stored  up  for  the 
growing  embryo.  These  same  variations  also  affect  the  later  stages  of 
development;  the  description  given  here  follows  the  simplest  conditions. 

As  a  result  of  segmentation  the  egg  is  converted  into  a  spherical 
mass  of  cells  in  which  a  cavity  appears,  called  the  segmentation 
cavity  because  it  is  formed  during  segmentation.  It  also  has  the 
name  archicoele  as  it  is  the  first  or  oldest  space  to  appear  in  the 
embryo.     This  stage  of  the  embryo  is  called  the  blastula  (fig.  3). 


EMBRYOLOGY.  9 

Its  cells  at  first  show  but  little  differentiation  except  in  size.  Next 
follow  processes  which  are  to  differentiate  the  cells  into  layers,  charac- 
terized by  both  position  and  fate. 

Gastrulation. — In  the  simplest  form  this  differentiation  is  brought 
about  by  an  inversion  of  one-half  of  the  blastula  into  the  other,  thus 
more  or  less  completely  obliterating  the  segmentation  cavity,  much  as 
one  may  push  one  side  of  a  rubber  ball  into  the  other,  forming  a  double- 
walled  cup  (fig.  4).  This  stage  is  called  the  gastrula,  and  the  process 
of  inpushing  fs  invagination.  With  this  the  first  appearance  of  the 
structures  of  the  adult  is  seen.  The  outer  wall  of  the  cup  is  turned^to 
the  external  world  and  thus  act  as  a  skin  for  the  embryo.  This  layer 
is  called  the  ectoderm.     The  opening  or  mouth  into  the  cup  is  the 


Fig.  3. — Diagram  of  a  typical 
blastula  with  central  segmentation 
cavity. 


b      sc  a      en 


ec 


Fig.  4. — Diagram  of  a  gastrula. 
a,  archenteron ;  b,  blastopore ;  ec,  ecto- 
derm; en,  entoderm;  5C,  segmentation 
cavitv. 


blastopore.  The  inside  of  the  cup  is  well  fitted  for  the  digestion  of 
food  as  it  can  be  held  together  there  and  the  digestive  fluids  are  less 
liable  to  waste.  Hence  the  cavity  is  called  the  archenteron  (primitive 
stomach),  and  the  layer  of  cells  which  line  it  is  the  entoderm.  That 
these  comparisons  are  more  than  analogies  of  position  is  shown  by. 
their  fates,  the  ectoderm  forming  part  of  the  skin  of  the  adult,  the 
entoderm  the  lining  of  the  digestive  tract.  Between  ectoderm  and 
entoderm  are  the  remains  of  the  segmentation  cavity,  filled  with  an 
albuminous  fluid.  It  will  be  convenient  later  to  speak  of  the  line  where 
ectoderm  and  entoderm  meet  at  the  blastopore  as  the  ect-ental  line. 
Closure  of  the  Blastopore. — Next,  the  blastopore  closes,  the 
process  beginning  at  what  will  be  the  head  end  of  the  embryo  and  pro- 


lO 


INTRODUCTION. 


ceeding  gradually  backward.  Usually  the  closure  is  complete,  but 
occasionally  the  hinder  part  remains  open  and  forms  the  anus.  Where 
it  closes  completely  the  vent  is  subsequently  formed  in  the  line  of  closure. 
This  union  of  the  two  lips  of  the  blastopore  in  closing  marks  the  middle 
line  of  the  back  of  the  future  animal,  and  is  called  at  first  the  primitive 
groove,  the  region  on  either  side  of  it  being  known  as  the  primitive 
streak,  terms  of  importance  in  understanding  the  gastrulation  of  the 
higher  vertebrates. 

Mesoderm. — With  the  closure  of  the  blastopore  the  embryo  elon- 
gates and  the  archenteron  is  converted  into  a  tube.  Next,  from  the 
region  of  closure  and  from  the  entodermal  tissue,  a  fold  of  cells  grows 
in  on  either  side  between  ectoderm  and  entoderm,  thus  farther  en- 
croaching on  the  segmentation  cavity.     These  cells  form  the  middle 


Fig.  5.  Fig.  6. 

Fig.  5. — Stereogram  of  the  anterior  end  of  a  developing  amphibian,  showing  the  out 
lining  of  the  mesothelium,  nervous  system  and  notochord.  a,  anterior  end ;  ar,  archenteron; 
c,  coelom;  ch,  notochordal  cells;  ec,  ectoderm;  mp,  mesodermal  pouch;  ng,  primitive  groove; 
np,  neural  plate;  nr,  neural  folds;  sc,  segmentation  cavity;  so,  somatic  wall  of  coelom;  sp^ 
splanchnic  wall  of  coelom. 

Fig.  6. — Stereogram  of  the  anterior  end  of  a  vertebrate,  shov^dng  the  relation  of  the 
coelomic  pouches;  c,  coelom;  d,  digestive  tract;  e,  ectoderm;  nc,  nervous  system; «,  notochord; 
sc,  segmentation  cavity;  so,  somatic  and  sp,  splanchnic  walls. 

layer  or  mesoderm.  Inside  this  fold  is  a  space,  connected  at  first  with 
the  archenteron,  but  soon  the  cavity  of  each  side  is  cut  off  by  a  growing 
together  of  the  opening  into  the  archenteron  and  is  henceforth  known 
as  a  coelom^  or  body  cavity.  Each  coelomic  space  has  two  walls,  one 
toward  the  ectoderm,  the  somatic  layer,  the  one  toward  the  entoderm 
being  the  splanchnic  layer  (figs.  5  and  6). 

The  mesoderm  arising  in  this  way  and  bounding  the  coelom  is 
called  mesothelium  to  distinguish  it  from  another  kind — the  mesen- 

^  A  ccelom  formed  in  this  way  is  an  enterocoele.  Usually  the  coelomic  walls  arise  as  a 
solid  mass  of  cells  from  the  corresponding  region,  which  later  splits  internally,  forming  a 
schizocoele.     The  two  are  readily  compared. 


EMBRYOLOGY.  1 1 

chyme — which  also  comes  to  lie  in  the  segmentation  cavity.  This 
mesenchyme  arises  as  separate  cells,  coming  largely  from  the  mesothe- 
lium,  and  to  a  less  extent  from  the  entoderm  (see  p.  i6).  Whether 
any  arises  from  the  ectoderm  is  disputed. 

The  Germ  Layers. — Ectoderm,  entoderm  and  the  two  types  of 
mesoderm  are  called  the  germ  layers,  because  in  the  animals  first 
studied  they  were  arranged  like  layers  one  on  the  other.  Each  plays 
its  part  in  the '  formation  of  the  adult  and  gives  rise  to  its  peculiar 
structures. 

The  ectoderm  forms  the  outer  layer  of  the  skin,  hair,  claws,  feathers, 
the  outer  layer  of  scales,  enamel  of  teeth,  and  the  essential  or  character- 
istic part  of  all  sensory  and  nervous  structures. 

The  entoderm  gives  rise  to  the  lining  of  the  digestive  tract,  and  the 
various  outgrowths — gills,  lungs,  liver,  pancreas,  etc. — connected  with 
it.  The  notochord  is  also  entodermal  and  possibly  the  lining  of  the 
blood-vessels  is  derived  from  this  layer. 

The  mesothelium  produces  the  lining  of  the  coelomic  cavities 
— pericardial,  pleural,  peritoneal — the  reproductive  and  excretory 
organs  and  the  voluntary  muscles  and  those  of  the  heart. 

The  mesenchyme  develops  the  deeper  layer  (corium)  of  the  skin 
and  of  scales,  the  dentine  of  teeth,  involuntary  muscles  (except  those  of 
the  heart)  connective  tissue,  ligaments,  cartilage,  bone,  and  the  corpus- 
cles of  blood  and  lymph. 

In  the  development  of  the  embryo  several  processes  of  differen- 
tiation occur  simultaneously,  but  in  the  written  account  one  has  to 
follow  another.  Hence  it  must  be  understood  that  the  modifications 
described  here  may  be  taking  place  at  the  same  time. 

The  Central  Nervous  System. — During  the  closure  of  the  blasto- 
pore the  ectoderm  in  front  and  to  either  side  of  the  blastoporal  lips 
becomes  thickened,  the  cells  elongating  at  right  angles  to  the  surface 
and  becoming  cylindrical  or  fusiform.  These  cells  form  the  neural  or 
medullary  plate  (fig.  5,  w/>),  sharply  marked  off  from  the  surrounding 
cells,  which  are  more  flattened,  and  which  eventually  are  concerned  in 
the  formation  of  the  outer  layer  (epidermis)  of  the  skin.  The  neural 
plate  is  to  develop  into  the  brain  and  the  spinal  cord,  and  it  is  to  be 
noted  that  later  it  extends  around  the  hinder  end  of  the  blastopore. 
After  it  is  outlined  the  plate  is  rolled  into  a  tube,  its  front  end  and  lateral 
margins  rising  up,  forming  neural  folds  (nr),  between  which  is  the 
medullary  groove.     Eventually  the  folds  meet  and  fuse  above  so  that 


12 


INTRODUCTION. 


the  tube  results  (fig.  6,  nc),  the  cavity  of  which  persists  throughout  life 
as  the  cavities  (ventricles)  of  the  brain  and  the  central  canal  of  the 
spinal  cord.  From  the  cells  of  the  walls  of  the  canal  the  nervous  tissue 
arises. 

This  process  of  infolding  progresses  from  in  front  backward.  For 
a  time,  in  some  vertebrates,  a  small  opening,  the  anterior  neuropore, 
persists  at  the  anterior  end.  The  infolding  extends  back  to  the  poste- 
rior end  of  the  neural  plate  so  that,  as  will  readily  be  understood,  the 
whole  limits  of  the  blastopore  are  included  in  the  floor  of  the  neural 
canal.  Occasionally  the  closure  of  the  neural  folds  is  completed  before 
that  of  the  blastopore  so  that  for  a  short  time  a  short  tube,  the  neuren- 
teric  canal  (fig.  7),  connects  the  archenteron  with  the  neural  canal. 
Soon  after  the  closure  of  the  neural  tube  the  fused  tissue  splits  horizont- 
ally, separating  the  nervous  sys- 
tem from  the  rest  of  the  ectoderm. 
Its  subsequent  history  will  be 
traced  in  the  section  of  the  Ner- 
vous System. 

The  Notochord. — Immediately 
beneath  the  neural  plate  is  an  axial 
strip    of    entoderm    (fig.    5,  ch)^ 

Fig.  7.-Schematic  section  of  the  hinder  bounded  On  either  side  of  the  OUt- 

end  of  an  amphibian  embiyo,  showing  the  growing  mesothclium.       When  the 

relations  of  the  neurenteric  canal,    ac,  alimen-  .             \     v'     i.       j 

tary  canal ;ec,  ectoderm  (black) ;«,  notochord;  latter   separates    (p.    lO)   thlS   band 

ne.  neurenteric  canal;  nt  neural  tube;  p,  is  momentarily  rejoined  tO  the  rCSt 
proctodeum;  pa,  post-anal  gut;  y,  yolk.  •'       •' 

of  the  entoderm  but  is  still  recogniz- 
able from  its  different  cells.  It  soon  rolls  into  a  rod  (a  tube  in  some 
amphibians  and  birds),  is  cut  off  from  the  rest  (fig.  6,  n)  and  lies 
between  the  digestive  tract  and  the  nervous  system  where  it  forms  an 
axis  around  which  the  skull  and  vertebral  column  develop  later. 

The  Digestive  Tract. — After  the  separation  of  the  notochord,  the 
entoderm  forms  a  tube,  closed  in  front  and  usually  behind  as  well. 
The  anterior  end  of  the  tube  abuts  against  the  ectoderm  of  the  ventral 
side  of  the  embryo.  Later  the  ectoderm  grows  in  at  the  point  of  con- 
tact, carrying  the  entoderm  before  it  and  forming  a  pocket,  the  stomo- 
deum,  which  gives  rise  to  the  cavity  of  the  mouth.  (In  some  the 
stomodeal  ingrowth  is  at  first  solid,  the  pocket  being  formed  later  by 
splitting).  Eventually  the  ectoderm  and  entoderm  fuse  at  the  bottom 
of  the  cup,  and  then  the  fused  area  breaks  through,  placing  the  archen- 


EMBRYOLOGY. 


13 


teron  in  connexion  with  the  exterior.  A  similar,  but  less  well  defined 
proctodeum  (fig.  7,  p)  arises  at  the  hinder  end  of  the  digestive  tract. 
Thus  the  anterior  and  posterior  ends  of  the  alimentary  canal  a^e 
ectodermal,  the  middle  region  entodermal,  in  origin. 

Metamerism. — In  the  adult,  various  parts,  essentially  like  each 
other,  are  repeated  one  after  another — are  metameric.  The  list 
includes,  among  others,  muscles,  nerves,  blood-vessels,  vertebrae,  ribs, 
etc.  There  is  much  evidence  to  show  that  metamerism  had  its  origin 
in  the  mesothelial  structures  and  has  been  secondarily  impressed  on 
other  systems. 


Fig.  8. — Stereogram  of  a  later  stage  than  fig.  6,  showing  the  segmentation  of  the  meso- 
thelium.  The  approach  of  the  walb  of  the  coelom  (c),  dorsal  and  ventral  to  the  alimentary 
canal,  to  form  the  mesenteries  is  shown,  d,  alimentary  canal;  em,  epimere;/6,  forebrain; 
/t6,  hind  brain;  hm,  hypomere;  m,  myotome;  nth,  midbrain;  mm,  mesomere;  mc,  metacoele; 
myc,  myocoele;  n,  nervous  system;  nc,  notochord;  s,  stomodeal  region;  so,  sp,  somatic  and 
splanchnic  layers;  st,  sclerotome. 

The  mesothelial  coelomic  pouches,  as  left  above,  are  near  the  dorsal 
side  of  the  embryo.  With  growth  they  gradually  extend  downward 
on  either  side  and  tend  to  enclose  the  whole  archenteron,  and  upward 
on  either  side  of  the  notochord  and  spinal  cord  (fig.  8).  The  fates  of 
the  different  parts  of  the  mesothelial  walls  warrants  the  recognition  of 
three  horizontal  regions  or  zones  in  the  walls  of  each  coelom.  These 
are  a  dorsal  muscle-plate  zone  (epimere,  em),  a  lower  or  lateral-plate 
zone  (hypomere,  hm),  and  a  middle-plate  zone  (mesomere,  mm) 
between  them.  All  three  of  these  occur  in  the  trunk,  but  only  the 
epimere  is  well  developed  in  the  anterior  part  of  the  head. 


14  INTRODUCTION. 

A  series  of  vertical  constrictions  begins  at  the  dorsal  margin  of  each 
coelomic  pouch  and  cuts  down  through  epimere  and  mesomere,  so 
that  the  whole  may  be  compared  to  a  glove  with  a  large  number  of 
fingers  extending  from  its  upper  surface,  each  finger  being  hollow,  and 
all  of  the  cavities  connecting  with  that  in  the  hypomere  (palm).  This 
process  begins  at  front  and  gradually  extends  backward.  Viewed 
from  above  in  the  transparent  embryo,  each  of  these  fingers  appears 
like  a  square  box  and  early  students  thought  that  they  gave  rise  to  the 
vertebrae,  and  so  they  were  called  proto vertebrae.  Next,  the  dorsal 
part  of  each  of  these  fingers  is  cut  off  from  the  rest,  along  the  line 
between  mesomere  and  epimere,  thus  forming  a  series  of  hollow  cubes, 
known  as  myotomes,  each  with  a  part  of  the  ccelom  in  its  interior,  the 
myocoele.  After  the  separation  from  the  rest  each  myotome  grows 
upward  and  to  a  greater  extent  downward,  insinuating  itself  between 
the  ectoderm  and  the  somatic  wall  of  the  hypomere  (fig.  9,  in  the 
direction  of  the  arrows).  From  these  myotomes  the  body  (somatic) 
musculature  arises. 

From  the  medial  mesomeral  part  of  the  fingers  arises  the  mesen- 
chyme that  gives  origin  to  the  vertebrae  while  the  rest  furnishes  the 
material  for  the  excretory  organs.  From  their  origin  both  of  these 
are  metameric  at  first,  the  skeletogenous  parts  being  called  scler- 
otomeis,  the  excretory  parts,  nephro tomes  (fig.  8,  mm,  st).  The 
history  of  both  will  be  followed  in  their  proper  places. 

The  Ccelom. — The  parts  of  the  coelom  in  the  myotomes  soon 
disappears,  that  in  the  nephrotomes,  of  inconsiderable  size,  forms 
the  lumina  of  the  excretory  ducts.  That  in  the  hypomere  (fig.  9,  c) 
forms  the  large  body  cavity  (peritoneal  cavity)  surrounding  the 
chief  viscera,  and  the  smaller  one  (pericardial)  around  the  heart. 
In  surrounding  the  archenteron  the  walls  of  the  two  coelomic  cavities, 
which  at  first  are  separate,  tend  to  meet  above  and  below  the  entoderm, 
so  that  there  is  in  both  Regions  a  thin  membrane  supporting  the  digest- 
ive tract  above  and  below.  Such  supports  are  collectively  called 
mesenteries.  Usually  that  below  {v  mes)  largely  disappears,  but  the 
dorsal  {d  mes)  one  persists  more  or  less  completely.  At  first  these 
mesenteries  are  merely  double  membranes  of  mesothelium,  but  soon 
mesenchyme  grows  in  between  them  and  extends  around  the  digestive 
tract,  so  that  mesothelium  and  entoderm  are  bound  together  by  the 
invading  tissue.  In  a  similar  way  the  somatic  wall  of  the  coelom  is 
bound  to  the  muscles  arising  from  the  myotomes  and  these  in  turn  to 


EMBRYOLOGY. 


15 


the  ectoderm  by  the  mesenchyme.  In  this  way  the  ccelom  comes  to 
have  two  thick  walls.  That  on  the  outer  side,  consisting  of  ectoderm, 
muscles  and  peritoneal  lining,  is  called  the  somatopleure  {so)f  that  of 
peritoneum  and  digestive  wall  is  the  splanchnopleure  (sp). 

For  convenience  the  different  mesenterial  structures  have  separate  names. 
As  the  digestive  tract  bfecomes  coiled,  the  different  parts  of  it  are  connected  by 
similar  membranes  which  are  called  omenta  (om).  The  dorsal  mesentery  is  sub- 
divided  into  regions  supporting  the  different  portions  of  the  digestive  tract. 


Fig.  9. — Diagrammatic  transverse  section  of  a  vertebrate  to  illustrate  mesenteries, 
omentum  and  downward  growth  of  the  myotomes,  al,  alimentary  tract;  ao,  aorta;  c, 
coelom;  ec,  ectoderm;  dmes,  dorsal  mesentery;  my,  myotome;  nc,  notochord;  neph,  nephro- 
tome;  o,  omentum;  sc,  spinal  cord;  so,  sp,  somatic  and  splanchnic  layers  of  mesothelium; 
vmes,  ventral  mesentery. 

Thus  there  is  a  mesogaster  for  the  stomach,  a  mesentery  proper  for  most  of  the 
intestine,  and  mesocolon  and  mesorectum  for  colon  and  rectum  respectively. 
On  the  ventral  side  there  is  a  mesohepar,  bounding  the  liver  to  the  ventral  body 
wall.  In  the  same  way  the  omenta  are  distributed  into  hepato-duodenal,  gas- 
tro-hepatic  (small  omentimi),  etc.,  while  in  mammals  there  is  a  great  omentum, 
a  double  fold  of  mesogaster  and  mesocolon  which  connects  the  stomach  with  the 
transverse  colon. 

Similar  folds  are  formed  in  connection  with  other  organs.     Thus  the  heart 


1 6  INTRODUCTION. 

for  a  time  is  bound  to  the  pericardial  walls  by  dorsal  and  ventral  mesocardia; 
there  is,  in  mammals,  a  mediastinum  between  the  two  pleural  cavities,  connect- 
ing the  pericardium  to  the  body  wall,  while  frequently  the  ovaries  and  the  testes 
project  into  the  ccelom,  carrying  the  peritoneum  with  them,  thus  giving  rise 
to  a  mesovarium  or  a  mesorchiimi,  according  to  the  sex. 

The  Mesenchyme  has  two  chief  places  of  origin.  One  is  from 
the  splanchnic  wall  of  the  segments  of  the  mesomere,  each  of  which 
is  the  centre  of  rapid  cell  proliferation  and  forms  the  sclerotome  (fig. 
8,  St)  J  since  some  cells  arising  from  it  are  concerned  in  the  formation 
of  the  axial  skeleton.  These  cells  pass  in  to  surround  the  notochord, 
and  upward  on  either  side  of  the  central  nervous  system  and  downward 
beside  the  alimentary  canal,  thus  forming  a  partition  between  the 
two  sides  of  the  body.  A  second  source  of  the  mesenchymatous  cells 
is  from  the  somatic  wall  of  each  myotome,  all  of  the  cells  of  which  are 
transformed  into  this  layer,  and  lie  immediately  beneath  the  ectoderm. 
Thus  there  is  a  complete  envelope  of  mesenchyme  around  the  whole 
body.  From  these  and  from  other  sources  the  mesenchyme  extends 
everywhere  in  the  remains  of  the  segmentation  cavity — between  the 
muscles  and  around  the  various  viscera — forming  a  framework  in 
which  the  products  of  all  the  other  layers  are  enveloped  (fig.  30).  This 
mesenchymatous  framework  has  great  importance  in  the  development 
of  the  skeleton  and  its  general  plan  will  be  described  in  connection  with 
the  skeletal  structures. 

HISTOLOGY. 

In  the  gastrula  the  cells  differ  from  each  other  chiefly  in  position, 
and  the  same  is  true  even  when  the  germ  layers  are  first  differentiated. 
As  development  goes  on  the  differences  between  the  various  groups  of 
cells  increase,  each  group  becoming  more  specialized  for  some  one 
purpose  and  losing  the  power  to  do  more  than  the  one  kind  of  work. 
For  community  of  work  cells  of  the  same  kind  become  associated  to- 
gether, the  result  being  tissues.  A  tissue  then  is  a  connected  mass  of 
cells  similar  in  appearance  and  function,  together  with  a  varying 
amount  of  intercellular  substance,  usually  formed  by  the  cells  them- 
selves. The  study  of  the  minute  structure  of  animals  and  especially 
of  the  tissues  is  the  province  of  histology. 

There  are  many  kinds  of  tissues,  only  a  few  of  which  need  mention 
here,  but  all  may  be  grouped  under  four  great  heads:  epithelial 


HISTOLOGY. 


17 


nervous,  muscular  and  connective  tissues;  the  members  of  each  group 
having  certain  fundamental  points  in  common. 

Epithelial  Tissues. 

Epithelia  are  the  covering  tissues,  and  occur  on  any  free  surface, 
internal  or  external,  of  the  body.  Both  comparative  anatomy  and 
embryology  show  them  to  be  the  primitive  tissues,  for  there  are  many 
lower  animals  which  are  made  up  entirely  of  epithelia,  while  in  the 
vertebrates  the  embryo  consists  solely  of  epithelia  until  the  mesenchyme 
appears.  Epithelia  may  come  from  any  of  the  germ  layers,  in  rare 
cases  (synovial  cavities)  even  from  mesenchyme. 


Fig.  10. — Epithelia:  A,  cubical;  B,  squamous;  C,  cylindrical;  D,  stratified  cylindrical, 
ciUated  at  E;  F,  stratified  squamous. 

The  character  of  epithelium  varies  according  to  the  character  of 
the  work  it  has  to  perform.  That  on  the  outside  of  the  body  is  largely 
protective,  hence  it  is  often  thickened  and  strengthened  in  different 
ways  to  afford  resistance  against  external  injuries.  In  other  places, 
as  glands,  it  has  to  elaborate  and  to  allow  the  passage  outward  of 
material  from  within.  In  the  body  cavity  and  in  the  blood-vessels 
it  has  merely  to  form  the  thinnest  of  coverings,  while  in  the  case  of 
sensory  structures  it  is  modified  (sensory  epithelium)  to  receive  the 
stimuli  from  without. 

The  usual  classification  of  epithelia  is  based  on  the  shapes  and 
arrangements  of  the  cells.  Thus  in  cubical  epithelium  (fig.  10,  A) 
the  cells  are  about  as  high  as  broad;  in  columnar  (C)  their  height 


i8 


INTRODUCTION. 


exceeds  their  diameter;  while  in  squamous  epithelium,  the  cells  are 
thin  and  fiat,  covering  the  largest  amount  of  surface  with  the  least 
amount  of  material  (B).  Sometimes  the  epithelial  cells  are  in  a  single 
layer,  forming  simple  epithelium  (A,  B,  C);  in  other  places  there  are 
several  layers— the  epithelium  is  stratified  (D,  E,  F). 

Frequently  epithelia,  usually  of  the  columnar  variety,  are  called  upon 
to  move  fluids  slowly;  then  the  free  surface  is  covered  with  minute 
vibratile  hairs  or  cilia  {E)  which  create  currents.  In  glandular 
epithelium  the  cells,  usually  cubical  or  columnar,  are  specialized  for 
the  elaboration  of  secretions  to  be  used  by  the  animal  or  of  waste  prod- 
ucts (excretions)  to  be  voided  from  the  body. 


Fig.  II. 


-Different  types  of  glands;  A,  to  D,  tubular;  E,  F,  acinous;  A,  simple;  B,  coiled; 
C-F,  branched. 


Glands. — The  chief  kinds  of  glands  may  be  mentioned  here.  All  have  for 
their  function  the  extraction  and  elaboration  of  certain  products  from  the  blood, 
consequently  they  have  a  good  blood  supply.  Glands  may  be  unicellular  or  multi- 
cellular according  as  they  consist  of  isolated  cells  or  of  many  cells.  In  unicellular 
glands  (abundant  in  the  digestive  tract)  each  cell  passes  its  own  secretion  directly 
to  the  place  where  it  is  to  be  used  (fig.  19,  u). 

Multicellular  glands  occur  where  a  large  amount  of  secretion  is  necessary  in  a 
limited  space,  hence  they  are  not  on  the  surface  but  at  some  deeper  point,  and  their 
product  is  conveyed  to  the  desired  place  by  a  duct.  Multicellular  glands  are  of  two 
structural  kinds.  In  the  tubular  gland  the  whole  is  approximately  of  the  same 
diameter  throughout,  with  little  differentiation  of  gland  and  duct.  It  may  be 
simple  (A)  or  coiled  (B)  or  branched  (C,  D),  these  modifications  serving  to  in- 
crease the  secreting  surface.  In  acinous  glands  (Z?,  E)  there  is  a  marked  differ 
ence  between  gland  and  duct,  the  glandular  part  forming  an  enlargement  (acinus) 
on  the  end  of  the  duct.     Both  simple  and  compound  acinous  glands  are  common. 

Still  another  type  of  gland,  the  ductless  or  'internal  secretion'  gland  occurs. 
In  this  there  is  no  duct,  the  secretion  elaborated  by  the  cells  passing  by  osmose  into 
the  blood-vessels.  These  secretions,  collectively  known  as  hormones,  have 
recently  acquired  great  prominence  from  their  influence  on  different  organs. 


HISTOLOGY. 


Nervous  Tissues. 


19 


Nervous  tissue  has  for  its^unction  the  correlation  of  the  animal  with 
its  environment.  In  order  to  accomplish  this  it  must  provide  for  the 
recognition  of  stimuli  from  without,  the  inauguration  of  other  impulses 
within  itself  and  the  transfer  of  both  to  other  parts.  The  essential 
constituent  of  the  tissue  is  the  nerve  cell,  ganglion  cell  or  neuron, 
to  which  are  added  others  of  a  supportive  (glia  cells)  or  nutritive 
character.  As  the  parts  to  be  connected  by  the  nervous  tissue  are  often 
remote  from  each  other  the  neuron  is  not  compact  like  most  other  cells, 
but  gives  off  long  processes  from  the  central  mass,  these  processes  differ- 
ing in  their  terminations.     Some  end  in  places  where  they  can  only 


Fig.  12. — ^V^arious  kinds  of  nerve  cells.  A,  multiJ)olar  cells;  B,  portion  of  nerve  fibre 
with  sheaths;  C,  unipolar  cell;  D,  pyramidal  cell;  a,  axon;  c,  collateral;  d,  dendrites;  cb, 
cell  body;  m,  medullary  sheath;  n,  nucleus  of  cell  of  Schwann's  sheath;  s,  sheath  of  Schwann; 
t,  telodendron. 


receive  stimuli,  others  where  the  stimuli  can  only  cause  parts  to  act. 
Thus  the  processes  are  physiologically  divisible  into  afferent  and 
efferent  tracts,  the  body  of  the  cell  being  the  place  for  the  regulation  and 
correlation  of  the  impulses,  and  apparently  in  many  cells  for  the  inau- 
guration of  new  impulses. 

A  nerve  cell  (fig.  12)  is  uni-,  bi-  or  multipolar  accordingly  as  it  has 
one,  two  or  more  of  these  processes.  In  the  case  of  unipolar  cells  (C) 
the  single  process  sooner  or  later  divides,  so  that  the  cell  in  reality  is  at 
least  bipolar.  At  the  ends  the  processes  may  either  break  up  in  minute 
twigs  (dendrites,  d)  or  may  end,  as  in  muscles  and  sense  organs,  in 
special  end  organs.  The  part  connecting  the  efferent  termination  and 
the  central  cell  body  is  called  the  axon  (a).  Axons  and  cell  bodies  are 
gray  in  color,  but  usually  the  axons  are  surrounded  by  a  medullary 
sheath  (m)  of  a  peculiar  white  substance  (myelin)  rich  in  fat,  which 


20  INTRODUCTION. 

apparently  acts  as  an  insulator,  preventing  nervous  impulses  from  pass- 
ing from  one  axon  to  another.  This  sheath  does  not  continue  over  the 
dendrites.  Frequently  the  dendrites  of  two  neurons  interlace  for  the 
transference  of  stimuli  from  one  to  the  other,  but  the  present  opinion  is 
that,  at  least  in  vertebrates,  there  is  no  actual  continuity  of  substance 
between  neurons,  only  an  interlacing  of  terminal  twigs.  The  medullary 
sheath  is  not  cellular,  but  frequently  fibres  may  be  surrounded  by  a 
sheath  of  Schwann  (s),  with  scattered  nuclei.  This  has  been  re- 
garded as  mesenchymatous,  but  recent  researches  tend -to  show  that 
it  is  ectodermal,  its  cells  coming  from  the  nervous  system. 

Nervous  tissue  consists  of  these  neurons  plus  connective  tissue  and 
glia  cells.  A  nerve,  as  found  in  dissection,  consists  of  numbers  of 
axons,  bound  together  by  a  connective-tissue  envelope  (perineureum) . 
The  myelin  gives  these  nerves  a  white  color.  In  the  brain  and  spinal 
cord  there  are  tracts  of  meduUated  fibres  (white  matter)  while  the 
parts  with  abundant  nerve  cells  are  gray.  When  such  gray  matter  is 
aggregated  in  the  course  of  a  nerve,  it  causes  an  enlargement  called  a 
ganglion.  Interlacing  among  the  neurons  in  brain  and  spinal  cord  is 
the  neuroglia,  which  is  also  derived  from  the  ectoderm,  and  acts  as  a 
support  but  has  no  nervous  functions.  Certain  of  these  glia  cells 
develop  many  branches  (mossy  cells)  which  twine  among  nervx  cells, 
axons,  and  dendrites. 

Muscular  Tissues. 

While  several  kinds  of  cells  have  the  power  of  changing  shape, 
those  composing  muscular  tissue  possess  it  in  a  marked  degree,  acting 
quickly  and  with  force,  so  that  these  tissues  are  preeminently  the  tissues 
of  motion.  The  cells  become  elongate  and  develop  on  their  interior  a 
large  amount  of  contractile  substance  (myofibrillae),  which  on  stimula- 
tion, contracts,  shortening  the  cell.  In  the  vertebrates,  muscular  tissue 
always  arises  from  the  mesoderm,  yet  two  types  are  recognized,  differing 
markedly  in  origin,  appearance  and  physiological  action. 

The  smooth  or  involuntary  muscles  arise  from  the  mesenchyme. 
They  consist  of  long  and  spindle-shaped  cells  (fig.  13,  A),  each  with  a 
single  nucleus,  the  protoplasm  traversed  by  numerous  myofibrillae, 
which  appear  like  fine  longitudinal  lines.  In  the  vertebrates  the 
smooth  muscle  is  not  under  control  of  the  will;  it  contracts  slowly. 

In  contrast  to  the  smooth  is  the  striped  or  voluntary  muscular  tis- 


HISTOLOGY.  2 1 

sue,  which  arises  from  a  modification  of  the  mesothelium.  Except  in 
the  case  of  the  muscles  of  theheart,  the  striped  tissue  is  under  control  of 
the  will;  it  usually  occurs  in  larger  masses  than  does  the  smooth,  and 
is  capable  of  rapid  contraction.  It  differs  structurally  from  smooth 
muscle.  Instead  of  distinct,  uninucleate  cells  there  are  long  cylindrical 
elements  (fig.  13,  B),  the  primitive  fibres,  each  with  several  nuclei  in  the 
interior  in  lower  vertebrates,  on  its  periphery  in  the  higher.  Most  of 
the  protoplasm  of  the  fibre  has  been  altered  to  minute  contractile 
fibrillae,  each  crossed  by  lighter  and  darker  bands,  and  as  these  come 
opposite  each  other  in  the  different  fibrillae,  they  give  the  fibre  its 
characteristic  cross-banded  appearance. 


Fig.  13. — A,  smooth  muscle  cell;  B,  striped  muscle. 

The  primitive  fibres  rarely  branch  at  their  extremities.  Each  is 
surrounded  by  a  structureless  envelope,  the  sarcolemma,  while  num- 
bers of  fibres  are  bound  into  bundles  and  muscles  by  connective 
tissue  (perimysiiun)  which  carries  nersTS  and  blood-vessels.  At  the 
ends  of  the  bundles  the  perimysium  continues  into  the  tendons  which 
attach  the  muscles  to  other  parts. 

The  heart  muscle  also  arises  from  the  mesothelium,  is  cross-banded, 
but  is  removed  from  control  of  the  will.  The  cells  are  usually  short 
(usually  with  a  single  nucleus) ;  they  branch,  the  branches  connecting 
adjacent  muscle  cells. 

Connective  Tissues. 

The  tissues  grouped  here  arise  from  the  mesenchyme  and  are 
distinguished  from  all  other  tissues  by  the  great  amount  of  intercellular 
substance  produced  by  the  cells  themselves.  This  substance  or  matrix 
varies  in  character  and  determines  the  variety  of  tissue.  Frequently  it 
is  dense  and  hence  the  connective  tissues  may  give  the  body  support, 
and  in  fact  they  are  sometimes  called  supportive  tissues. 


22 


INTRODUCTION. 


In  the  earliest  phase,  known  as  embryonic  connective  tissue 
(fig.  14,  ^),  the  cells  are  scattered,  with  long  radiating  processes,  and 
between  the  cells  a  thin  gelatinous  matter.  It  is  by  increase  of  this 
intercellular  substance  by  taking  up  water  that  many  embryos  gain  so  in 
size  without  taking  food.  The  embryonic  connective  tissue  may  de- 
velop in  various  directions. 


^^^ 


Fig.  14. — Connective  tissues.     A,  embryonic,  from  Amhly stoma;  B,  expanded  and  con- 
tracted pigment  cells  from  Amhly  stoma;  C,  fibrous,  from  tendon. 

Thus  some  of  the  cells  may  contain  pigment  granules,  forming 
pigment  cells  (J5),  or  oil  globules  may  be  deposited  in  them  to  such  an 
extent  that  the  cells  become  spherical,  while  the  intercellular  substance 
is  reduced,  thus  affording  fat  or  adipose  tissue.  Most  common  of 
the  connective  tissues  is  fibrous  tissue  (white  or  non-elastic  tissue)  in 
which  the  cells  are  branched  or  spindle-shaped  while  the  matrix  is 
filled  with  fine  fibrillae  of  considerable  strength  and  little  elasticity. 

These  fibrillae  are  parallel  to  each  other  in  tendons  (C),  which  have  to 
convey  strains  in  one  direction;  or  they  maybe  interlaced  confusedly,  the 
tissue  then  forming  sheets  or  membranes.  Occasionally,  as  between 
the  skin  and  the  muscles,  the  fibrous  tissue  may  be  loose  (areolar 
tissue).  In  elastic  tissue  fibres  of  another  kind  are  mingled  among 
the  non-elastic  fibrils.  These  are  yellow  and  elastic,  and  when  abun- 
dant give  an  elastic  character  to  the  whole. 

In  cartilage  and  bone  the  matrix  is  more  solid  and  is  abundant. 
These  are  the  skeleton-building  tissues.     In  cartilage  the  matrix  is 


HISTOLOGY. 


23 


firm  and  consists  of  a  peculiar  substance  called  chondrin.  When  the 
chondrin  is  nearly  pure  it  is  milky  in  appearance  (hyaline  cartilage, 
fig.  15),  but  it  may  be  invaded  by  numerous  strands  of  fibrous  or 
elastic  tissue,  resulting  in  'fibrous  or  elastic  cartilage.  Cartilage  in- 
creases in  size  by  additions  to  the  exterior  and  also  by  divisions  of 
its  cells  and  by  increase  in  the  amount  of  matrix.     Externally  it  is 


Fig.  15. — Hyaline  cartilage. 

bounded  by  an  envelope  of  connective  tissue  (perichondrium)  which 
bears  blood-vessels  and  may  give  attachment  to  muscles,  etc. 

Bone  may  arise  directly  from  embryonic  connective  or  fibrous  tissue, 
or  by  the  ossification  of  cartilage.  In  either  case  the  result  is  a  strong 
matrix  composed  of  calcium  phosphate  and  carbonate  in  a  ground 


•  » - «.  *  \ 


'^y)Pn\ 


Fig.  16. — A,  Stereogram  of  bone;  B,  cross-section  of  bone,  more  enlarged;  c,  canaliculi; 
bl,  bone  lamellae;  h,  Haversian  canal;  /,  lacvma. 

substance  of  organic  matter  (ossein).  Minute  tubes  (Haversian 
canals) ,  bearing  blood-vessels,  etc.,  run  through  the  matrix  (fig.  16), 
and  parallel  to  these  canals  or  to  the  external  surface  of  the  bone  are 
the  cells  arranged  in  layers.  The  space  occupied  by  a  cell  is  called 
a  lacuna,  from  which  minute  tubules  or  canaliculi  penetrate  the  matrix. 
There  are  small  spaces  in  many  bones  occupied  by  the  red  marrow, 


24  INTRODUCTION. 

which  is  especially  noticeable  as  one  of  the  places  of  formation  of  red 
blood-corpuscles.  Externally  every  bone  is  covered  by  a  layer  of 
fibrous  connective  tissue,  the  periosteum.  , 

The  dentine  of  teeth  and  placoid  scales  is  closely  allied  to  bone, 
the  chief  difference  in  density,  the  bone-forming  cells  (odontoblasts) 
not  being  enclosed  in  the  matrix,  while  the  canaliculi  (here  called  den- 
tinal canals)  are  parallel  to  each  other. 

Blood  is  sometimes  regarded  as  a  connective  tissue,  the  corpuscles 
being  the  cells  and  the  fluid  part  (plasma)  the  matrix.  It  is  here  dealt 
with  in  connection  with  the  circulatory  system. 


COMPARATIVE  MORPHOLOGY  OF 
VERTEBRATES. 

THE  INTEGUMENT. 

The  integument  is  the  covering  of  the  body,  the  term  including 
the  skin  (cutis)  and  all  structures  derived  from  it.  From  its  position 
it  is  a  protective  coat.  It  comes  into  relation  with  the  external 
world  and  is  modified  in  various  w^ays,  becoming  hardened  to  ward 
against  mechanical  injury,  developing  sensory  structures  to  give  in- 
formation of  untoward  conditions  and  being  impervious  so  as  to  prevent 
loss  of  the  body  fluids  or  the  entrance  of  others  from  without.  Natur- 
ally the  habitat,  aquatic  or  terrestrial,  has  great  influence  in  the 
character  of  the  modifications. 

In  all  vertebrates  the  integument  consists  of  two  layers,  an  outer 
epidermis  which  consists  of  the  ectoderm  after  the  separation  of  the 
nerv^ous  system,  and  a  deeper  layer,  the  cerium  (derma)  of  mesenchyme, 
derived  from  the  somatic  wall  of  the  myotomes,  into  which  other  struc- 
tures (nen-es,  blood-vessels,  etc.)  extend.  Strictly  speaking  the 
bony  scales  of  fishes  are  integumental,  but  on  account  of  their  close 
relations  to  the  skeleton  they  are  best  treated  in  that  connexion. 

In  the  epidermis,  again,  two  layers  are  always  present.  At  the  base, 
next  to  the  corium  is  theMalpighian  layer  (stratiun  germinativum) , 
the  cells  of  which  are  nourished  by  the  fluids  of  the  corium.  Hence 
they  can  grow  and  divide,  the  new  cells  thus  formed  gradually  passing 
to  the  outside  where  they  form  the  second  layer,  the  stratum  corneum, 
the  outer  cells  of  which  are  usually  worn  away  as  fast  as  new  ones  are 
added  from  below.  Occasionally  these  outer  cells  come  off  in  large 
sheets,  as  when  a  salamander  or  a  snake  sloughs  its  *skin.'  In  the 
development  of  the  epidermis  of  the  terrestrial  vertebrates  the  first 
layer  of  cells  budded  from  the  Malpighian  stratum  form  a  continuous 
sheet  which  is  later  shed  as  a  whole.  This  is  the  periderm  (fig.  17), 
the  older  name  of  epitrichivmi  being  inappropriate,  since  the  layer  is 
found  in  reptiles  and  birds  where  no  hair  occurs. 

25 


26         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

The  Malpighian  layer  alone  is  concerned  in  the  formation  of  the 
glands  connected  with  the  skin,  and  the  corresponding  part  of  the 
ectoderm  contributes  to  the  sensory  structures  like  the  nose  and  ear. 
The  corneum,  on  the  other  hand,  is  concerned  in  the  formation  of 
protective  structures  like  hair,  nails,  claws,  feathers,  and  other  cuticular 
outgrowths.  The  epidermis  is  generally  thicker  in  terrestrial  than  in 
aquatic  vertebrates,  and  in  the  latter,  being  constantly  moist,  shows 
less  of  the  horny  consistency,  than  occurs  in  animals  which  live  in  the 
air. 

The  corium  lies  immediately  beneath  the  epidermis  and  is  less 
sharply  separated  from  the  deeper  tissues  by  a  looser  layer  of  connective 
tissue  (subcutis,  tela  subjunctiva)  in  which  fat  is  frequendy  exten- 
sively developed.  The  corium  is  largely  composed  of  fibrous  connec- 
tive tissue,  intermingled  with  elastic  tissue,  blood-vessels,  nerves, 
smooth  muscle  fibres,  etc.    It  is  usually  thin  in  the  lower  vertebrates. 


Fig.  17. — Section  of  developing  scales  of  lizard,  Sceleporus.  c,  papilla  of  corium; 
e,  outer  layer  of  epidermis  which  later  becomes  cornified;/,  fibrous  layer  of  skin;  m,  Mal- 
pighian layer;  p,  periderm;  ts,  tela  subjunctiva. 

but  is  much  thicker  in  most  mammals,  and  forms  the  whole  of  ordinary 
leather.  Pigment  cells  may  occur  in  both  epidermis  and  corium.  These 
are  mesenchyme  cells,  loaded  with  pigment,  which  are  frequently 
under  control  of  the  nervous  (sympathetic)  system,  and  can  be  altered 
in  shape  (chromatophores) ,  thus  producing  color  changes,  which,  as 
in  the  chameleons,  may  be  very  marked. 

Horny  scales,  produced  by  a  cornification  of  the  epidermis,  are  found 
in  all  groups  of  terrestrial  vertebrates,  but  they  are  rare  in  amphibians 
and  mammals.  The  development  is  best  seen  in  reptiles  (fig.  17). 
By  a  multiplication  of  the  cells  of  both  corium  and  epidermis  in  defi- 
nite regions  the  skin  becomes  divided  into  thicker  areas,  separated 
by  thinner  lines,  each  area  corresponding  to  a  future  scale,  which  arises 
by  the  conversion  of  the  stratum  corneum  into  horny  material.  In 
snakes  and  lizards  these  scales,  together  with  all  of  the  stratum  corneum 
(even  the  covering  of  the  eye)  is  periodically  molted,  the  separation  tak- 


INTEGUMENT. 


27 


ing  place  at  the  surface  of  the  stratum  Malpighii.     In  turtles  and 
alligators  there  is  a  gradual  wearing  away  of  the  surface. 

Closely  allied  to  scales  are  claws,  hoofs  and  nails  (fig.  18).  A 
claw  may  be  regarded  as  a  cap  of  the  tip  of  a  digit,  formed  by  two  scales 
one  dorsal  (unguis),  the  other  ventral  (subunguis).  Of  these  the 
unguis  is  the  more  important.  It  grows  continually  from  a  root,  and  in 
mammals  is  forced  forward  over  its  bed.  In  the  claw  (B)  the  unguis 
is  curved  both  transversely  and  longitudinally,  the  subunguis  forming 
its  lower  surface.  In  the  human  nail  (A)  it  is  nearly  flat  in  both  direc- 
tions and  the  subunguis  is  reduced  to  a  narrow  plate  just  beneath  the- 


Fig.  18. — Diagrams  of  (A)  nails,  (B)  claws,  and  (C)  hoofs,  based  on  Boas,     e,  unmodified 
epidermis;  n,   unguis;  s,  subunguis. 


tip  of  the  nail.  In  the  hoof  (C)  the  unguis  is  rolled  around  the  tip  of  the 
toe,  while  the  subunguis  forms  the  'sole'  inside  it.  The  'frog'  is 
the  reduced  ball  of  the  toe  which  projects  into  the  hoof  from  behind. 

The  integument  presents  many  diJBFerent  conditions  in  the  separate 
groups  of  vertebrates,  and  so  details  are  best  given  under  the  special 
heads. 

FISHES. — The  aquatic  life  renders  the  epidermis  of  fishes  soft  and 
cornifications  of  it  are  comparatively  rare,  among  them  the  peculiar 
'pearl  organs'  which  appear  in  the  skin  of  some  teleosts  at  the  breed- 
ing season.  Glands,  on  the  other  hand,  are  abundant.  These  are 
unicellular  and  multicellular  mucus  glands  of  different  shapes  in  the 
epidermis,  the  secretion  of  which  furnishes  the  slime  on  the  surface. 
Some  elasmobranchs  and  a  number  of  teleosts  have  poison  glands, 
usually  in  close  relation  to  the  spines  of  the  fins.  The  elasmobranchs 
also  have  large  glands  in  the  'claspers'  of  the  males,  but  their  purpose 
is  not  well  understood. 


28 


COMPARATIVE  MORPHOLOGY  OF  VERTEBR.4TES. 


Possibly  the  most  striking  of  the  epidermal  organs  are  the  luminous 
organs  or  photophores,  which  are  most  common  in  elasmobranchs  and 
teleosts  from  the  deep  seas,  where  sunlight  does  not  exist.  They  are 
apparently  modified  glands,  and  the  development  is  known  in  Porich- 


FiG.  19. — Section  of  skin  of  Protopterus.     c,  corium;  e,  epidermis;  ^,  multicellular  gland; 

M,  unicellular  gland. 


thys.  There  is  an  involution  of  cells  of  the  Malpighian  layer  into  the 
corium,  where  they  become  cut  off  from  their  point  of  origin,  and  are 
differentiated  into  a  deeper  glandular  layer  and  an  outer  rounded  body, 
the  lens  (fig.  21).     Around  this  the  corium  forms  a  reflecting  layer 


Fig.  20. — A,  head  of  Noturus  flavus;  B,  section  of  poison  gland   of  Schilheodes  miurus 
(after  Reed),     e,  epidermis;  p,  pore  of  poison  gland,  pg;  s,  spine  of  pectoral  fin. 

enclosed  in  a  pigment  coat.  The  glandular  layer  is  the  seat  of  light 
production.  In  other  photophores  either  reflector  or  pigment  may  be 
lacking,  but  in  their  highest  development  they  so  resemble  an  eye  that 
at  first  they  were  described  as  such. 

In  the  myxinoids  the  skin  contains  numerous  thread  cells  in  pockets  which  may 
extend  into  the  underlying  muscles.     Each  thread  cell  contains  a  long  thread,  which 


INTEGUMENT. 


29 


is  discharged  upon  stimulation,  the  threads  forming  a  network  in  which  the  mucus 
secreted  by  the  ordinary  gland  cells  is  entangled. 

The  corium  is  thin  and  consists  of  horizontal  bands  of  fibrous  tissue,  crossed  at 
intervals  by  vertical  strands.  Fat  is  common  in  the  tela  subcutanea,  and  in  some 
fishes  this  layer  contains  numerous  crystals  of  guanin  which  gives  it  a  silvery 
appearance.  This  guanin  forms  the  base  of  'essence  of  pearl'  from  which  artificial 
pearls  are  made.  The  scales  of  fishes,  although  formed  in  the  skin,  are  con- 
sidered in  connection  vsith  the  skeleton. 


Fig.  21. — Section  of  luminous  organ  (photophore)  of  Porichthys,  after  Greene,  e, 
epidermis  \\-ith  mucous  cells;  gl,  glandular  layer  of  photophore;  /,  lens;  r,  reflector  sur- 
rounded by  pigment. 

AMPHIBIA. — The  amphibia  are  remarkable  in  that  the  epidermis 
of  the  larvae  is  ciliated  in  the  early  stages,  and  is  two  cells  in  thickness 
from  the  first.  The  skin,  in  the  larvae  and  the  aquatic  species,  con- 
tains numerous  mucus  glands  and  some  for  the  production  of  poison, 
some  of  the  latter  being  prominent  like  the  'parotid  glands'  on  the 
neck'of  the  anura  and  the  gland  on  the  back  near  the  base  of  the  tail. 

The  corium  is  thin,  and  in  the  frogs  is  separated  from  the  underlying  parts  by 
large  lymph  spaces  which  render  the  skinning  of  these  animals  so  easy.  As  the 
amphibia  respire  largely  by  the  skin  (there  are  several  lungless  salamanders)  the 
corium  is  richly  supplied  with  blood-vessels,  and  at  the  time  of  the  metamorphosis 
of  the  anura  these  penetrate  even  into  the  epidermis,  as  at  that  time  the  lungs  are 
not  yet  functional  and  the  gills  are  absorbed.  The  stratum  comeum  is  shed 
periodically,  either  as  a  whole  (urodeles)  or  in  patches.  The  warts  of  toads  are  in 
part  cornifications  of  the  epidermis,  and  a  similar  hardening  of  the  skin  on  the 
ends  of  the  toes  of  some  results  in  claws.  In  the  mates  of  an  African  frog 
(Astylosternus)  the  skin  has  the  granules  of  the  surface  developed,  at  the  breed- 


so 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


ing   season   into   hair-like   structures,   supplied   with   nerves   and   apparently 
sensory  in  character. 

REPTILES. — ^All  living  reptiles  are  characterized  by  the  extensive 
development  of  horny  scales  and  frequently  of  bony  plates  in  the  skin, 
but  some  of  the  fossil  groups  (ichthyosaurs,  pterodactyls,  some  dino- 
saurs, possibly  plesiosaurs)  had  a  naked  skin.  Correlated  with  this 
cornification  of  the  epidermis,  glands  are  rare.  Some  turtles  have  scent 
glands  beneath  the  lower  jaw  and  along  the  line  between  carapace  and 
plastron;  snakes  and  crocodilians  have  them  connected  with  the  cloaca, 
while  the  latter  have  others,  of  unknown  function,  between  the  first  and 
second  rows  of  plates  along  the  back,  as  well  as  protrusible  musk 
glands  on  the  lower  jaw.  These  latter  are  not  true  glands  as  they 
produce  no  secretion  but  cast  out  the  lining  cells. 

The  corium  presents  two  layers,  the  outer  rich  in  chromatophores,  but,  aside 
from  some  snakes  and  lizards,  the  color  changes  are  not  remarkable.  The  femoral 
pores  of  lizards  are  not  connected  with  glands  but  with  branching  tubes  filled  with 
cast  cells.     Claws  are  common  on  the  toes. 

BIRDS  have  both  layers  of  the  skin  very  thin,  the  epidermis  develop- 
ing both  scales  and  feathers.     Correlated  with  this  extensive  develop- 


FiG.  22. — Diagram  of  base  of  contour  feather,  a,  aftershaft;  b,  barbs;  bl,  barbules; 
h,  hooks  on  ends  of  barbules;  lu,  lower  umbilicus;  q,  quill;  s,  shaft;  u,  umbilicus;  v,  vane. 
A,  portion  of  a  barb  showing  the  barbules  and  hooks. 

ment  of  cornified  structures  is  a  striking  paucity  of  glands.  There  are 
none  in  the  ostriches,  but  others  have  the  familar  oil  (uropygial)  glands 
at  the  base  of  the  tail,  the  secretion  of  which  is  used  in  dressing  the 
feathers.     The  only  other  dermal  glands  in  birds  are  modified  sebaceous 


INTEGUMENT. 


31 


glands  near  the  ear  in  some  rasores.  The  scales  on  the  legs  and  the 
claws  on  the  feet  and  occasionally  on  the  wings,  are  derivatives  from 
reptilian  ancestors.  The  feathers  are  also  derived  from  scales,  but  are 
greatly  modified. 

Feathers. — There  are  several  kinds  of  feathers  but  all  may  be 
grouped  under  three  heads:  hair  feathers  (filoplumes),  down  feathers 
(plumulae),  and  contour  feathers  (plumae).  The  latter  have  all  of  the 
feather  features  (fig.  22)  and  in  the  typical  form  consist  of  shaft  and 
vane.     The  basal  part  of  the  shaft  is  the  hollow  quill,,  in  which  is  a 


Fig.  23. — ^Feather  tracts  of  Geococcyx  cali/omianus,  after  Shufeldt. 

small  amount  of  loose  pith.  In  the  region  of  the  vane  the  shaft,  here 
called  rhachis,  is  solid,  and  running  the  length  of  its  lower  surface  is  a 
groove,  the  umbilicus.  The  vane  consists  of  lateral  branches  (barbs) 
on  either  side,  which  have,  in  turn,  smaller  side  branches  (barbules), 
these  with  small  hooks  at  their  sides  and  tips  (B).  Interlocking  of 
these  hooks  gives  firmness  and  continuity  to  the  whole  vane.  In  down 
feathers  the  barbs  arise  directly  from  the  end  of  the  quill,  and  as  hooks 
are  lacking,  the  barbs  do  not  interlock  and  no  vane  is  formed.  Hair 
feathers  are  merely  long  and  slender  shafts  with  no  barbs,  the  simplest, 
if  not  the  most  primitive  kind  of  feather.     It  is  still  a  question  as  to 


32         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  primitive  type.     The  oldest  fossil  bird,  ArchcBOpteryx,  had  well 
developed  contour  feathers. 

Except  in  the  ostriches,  penguins,  and  toucans,  feathers  are  not 
distributed  everywhere  on  the  surface  of  the  body,  but  are  gathered  in 
feather  tracts  (pterylae),  separated  by  apteria  in  which  no  contour 
feathers  and  but  few  down  or  hair  feathers  occur.  These  vary  in  their 
arrangement  in  different  groups  of  birds  and  are  of  systematic  im- 
portance (fig.  23). 

Complicated  as  they  are,  feathers  are  probably  derived  from  scales,  and  the 
section  of  lizard  skin  (fig.  17)  might  well  represent  an  early  stage  in  the  develop- 
ment of  a  feather.  A  down  feather  begins  as  a  thickening  of  the  corium,  pushing 
the  epidermis  before  it.     By  continued  growth  this  forms  a  long,  finger-like  papilla. 


Fig.  24. — Stereogram  of  developing  down  feather,  hv,  blood-vessels  entering  pulp; 
c,  corium;  ep,  epidermis;  /,  feather  follicle;  f,  pulp  (mesenchyme)  of  developing  feather; 
per,  periderm;  r,  rods  of  epidermis,  which  later  dry,  separate,  and  form  the  down. 

projecting  from  the  skin.  The  corium  extends  into  the  outgrowth,  carrying  blood- 
vessels with  it,  while  an  annular  pit,  the  beginning  of  the  feather  follicle,  forms 
around  the  base  of  the  papilla.  Next,  the  corium  or  pulp  of  the  distal  part  of  the 
papilla  forms  several  longitudinal  ridges  (fig.  24)  which  gradually  increase  in 
height,  growing  into  the  epidermis  and  pressing  the  Malpighian  layer  above  them 
against  the  periderm.  As  a  result  the  stratum  comeum  is  divided  distally  into  a 
number  of  slender  rods  arising  from  the  base  (quill),  which  at  last  are  only  held 
together  by  the  periderm.  Then  the  pulp  retracts,  carrying  with  it  the  Mal- 
pighian layer.  With  the  blood  supply  removed,  the  epidermal  parts  dry  rapidly, 
the  periderm  ruptures,  allowing  the  rods  to  separate  to  form  the  down. 

A  contour  feather  has  much  the  same  development,  differing  in  details,  for  an 
account  of  which  reference  must  be  made  to  special  papers.  The  ridges  of  the 
corium  are  no  longer  longitudinal,  but  beginning  on  the  dorsal  side  of  the  papilla, 
run   obliquely  outward  and  downward  (fig.  25)  until  they  meet   below.     Thus 


INTEGUMENT.  33 

there  are  formed  a  series  of  rods  set  at  an  acute  angle  to  the  undivided  dorsal  strip, 
the  future  shaft.  When  set  free,  as  before,  by  the  rupture  of  the  periderm,  these 
rods  straighten  out,  forming  the  vane.  In  the  region  of  the  shaft  there  are  two 
longitudinal  ridges  on  the  ventral  side.  These  gradually  roll  together,  thickening 
and  strengthening  the  shaft,  the  groove  between  them  forming  the  umbilicus.  As 
will  be  understood,  the  dorsal  and  ventral  sides  of  the  feather  were  the  outside  and 
inside  of  the  stratum  comeum  of  the  papilla. 

The  corium  is  thin  and  consists  of  irregularly  interlaced  fibres;  it  is  rich  in  sense 
(tactile)  organs  and  smooth  muscle  fibres,  which  are  largely  used  in  elevating  the 
feathers.  The  colors  of  feathers  depend  in  part  upon  pigment — red,  yellow,  orange, 
brown,  and  black — deposited  in  them,  but  the  iridescent  colors  are  due  to  interfer- 
ence spectra. 


.M  .,-,.. 


Fig.  25. — Stereogram  of  part  of  developing   contour  feather;  compare  with  fig.  24.     b 
developing  barbs;  pc,  pith  cavity;  per,  periderm;  s,  rhachis. 

MAMMALS  have  a  skin  relatively  thicker  than  have  other  verte- 
brates, both  layers  contributing  to  the  thickness  and  the  whole  rather 
loosely  attached  to  the  lower  tissues.  There  are  numerous  glands,  and 
the  hair,  abundant  in  all  orders  except  the  whales  and  sirenians,  is 
found  in  no  other  class.  Other  cuticular  structures  as  horn  and  claws 
(p.  27)  are  widely  distributed  and  scales  occur  in  several  forms. 

The  corium  is  thick  and  composed  of  irregularly  interlaced  fibres  with  muscles, 
blood-vessels,  etc.  Its  outer  surface  is  frequently  thrown  into  papillae  or  ridges, 
especially  on  the  palms  and  soles,  these  carrying  the  epidermis  with  them.  In  the 
thick  epidermis  several  strata  may  usually  be  recognized:  at  the  base  a  thick 
Malpighian  layer;  then  a  thin  stratum  lucidum  in  which  distinct  cells  cannot  be 
recognized;  and  on  the  outside  the  stratum  comeum.  One  or  more  others  are 
sometimes  present.  As  w411  readily  be  understood  a  cell  passes  through  all  of 
these  layers  before  it  is  worn  from  the  surface  of  the  skin. 

Hair. — The  epidermis  takes  the  initiative  in  the  formation  of  hair. 

It  thickens  in  spots,  the  thickenings  pushing  into  the  corium  and  each 

being  cupped  at  the  tip,  blood-vessels  extending  into  the  cup.    The  basal 

cells  of  the  ingrowth,  thus  richly  nourished,  proliferate  rapiidly  and  the 

3 


34         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

new  cells  thus  formed  are  forced  outward,  forming  the  hair.  While 
this  is  going  on  the  ingrowth  splits  around  the  hair,  forming  the  follicle, 
while  another  ingrowth  of  the  Malpighian  layer  forms  the  sebaceous 
gland  which  oils  the  hair. 

A  section  through  a  hair  and  its  follicle  gives  the  following  layers  (fig.  26). 
Around  all  is  the  connective-tissue  envelope,  formed  from  the  corium;  next  inside 
is  the  outer  root  sheath  formed  of  the  Malpighian  layer  and  extending  to  the  cavity 
of  the  follicle.  Around  the  root  of  the  hair  is  the  inner  root  sheath,  two  cells  in 
thickness,  the  layers  being  known  as  Henle's  and  Huxley's  layers.  These  do  not 
extend  outside  the  follicle.  In  the  hair  itself  there  is  a  cortical  layer  surrounding 
the  central  medulla,  the  hair  not  being  hollow. 


Fig.  26. — Diagram  of  structure  of  hair,  h,  blood-vessels;  c/,  cuticle  of  hair;  ex,  cortex  g, 
gland;  h,  hair;  he,  Henle's  layer;  hf,  hair  follicle;  hx,  Huxley's  layer;  m,  medulla;  p,  papilla; 
sg,  stratum  germinativum  of  epidermis. 

Hair  differs  greatly  in  size,  the  spines  of  the  porcupines  forming  one  extreme,  the 
prenatal  hair  (lanugo)  of  man  the  other.  Hair  is  shed  at  intervals.  The  old  hair 
ceases  to  grow,  separates  from  its  base,  and  later  is  pushed  out  when  the  root  begins 
again  to  proliferate.  There  are  smooth  muscle  fibres  connected  with  the  roots  of 
the  hairs,  their  function  being  to  raise  the  hair  from  its  usual  inclined  position  under 
influence  of  the  sympathetic  system.  There  are  also  usually  nerves  distributed 
to  the  base  of  the  hairs,  making  them  to  some  extent  sense  organs,  a  condition 
which  reaches  its  greatest  development  in  the  facial  hairs  (vibrissae)  of  carnivores 
and  the  hairs  on  the  wings  of  bats. 

Scales  occur  in  several  orders,  being  usually  best  developed  on  the 
tail  and  feet.  They  may  be  rounded,  quadrangular  or  hexagonal,  the 
square  scales  being  arranged  in  rings  around  the  part,  the  others  in 
quincunx.  These  are  closely  similar  to  the  cuticular  scales  of  reptiles 
(p.  26).     Recent  investigations  tend  to  show  that  there  is  a  close  rela- 


INTEGUMENT. 


35 


tion  between  scales  and  hairs,  since  in  the  mammals  with  scales  the 
hairs  are  usually  arranged  in  groups  of  three  or  five  behind  each  scale 
(fig.  27) ;  while  in  those  without  scales  the  hairs  are  frequently  grouped 
in  the  same  manner.  The  illustration  (fig.  28)  is  interesting  as 
showing  the  arrangement  in  man  and  the  possible  relation  to  ancestral 


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Fig.  27. — A,  arrangement  of  the  two  kinds  of  hair  in  Ornithorhynchus;  B,  Arrangement 
of  hair  in  Ptilocerus  lori,  with  the  probable  relation  of  the  hair  to  the  ancestral  scales; 
both  after  Meijere. 


Fig.  28. — Arrangement  of  the  hairs  in  groups  of  threes  and  fives  in  the  human  embryo, 
with  the  probable  ancestral  arrangement  of  the  scales;  after  Stohr. 


scales.     The  statement  is  also  made  that  at  first  the  hairs  are  arranged 
in  longitudinal  rows  and  that  the  grouping  comes  later. 

The  mammalian  skin  is  usually  rich  in  glands  which  are  of  two 
types,  tubular  and  acinous  (p.  18) .  To  the  first  belong  the  sweat  glands, 
which  extend  from  the  Malpighian  layer,  where  they  arise,  down  through 


36 


COMPARA.TIVE   MORPHOLOGY    OF   VERTEBRATES. 


the  corium  and  then  are  coiled  in  order  to  obtain  greater  length.  The 
acinous  glands  are  represented  by  the  sebaceous  glands  in  connection 
with  each  hair  (fig.  26,  g),  and  by  the  scent  glands  in  the  anal  or  in- 
guinal region  of  many  carnivores,  rodents  and  edentates.  Others 
may  occur  in  very  diverse  regions  as  on  the  face  (bats,  deer),  in 
the  occipital  (camel)  or  temporal  region  (elephant)  or  on  the  legs 
(swine). 

The  mammary  or  milk  glands  are  now  known  to  be  modified  tubu- 
lar glands  possibly  derived  from  sweat  glands.  In  the  monotremes  the 
simplest  condition  is  found,  numbers  of  glands  opening  into  a  pair  of 
sacs  in  the  sides  of  the  marsupium,  or  pouch  where  the  young  are  kept, 


Fig.  29. — Scheme  of  different  kinds  of  nipples,  based  on  figures  by  Weber.  Single 
line,  ordinary  integument,  double  line,  that  of  primary  mammary  pocket.  A,  primitive 
condition,  found  in  Echidna;  B,  human  nipple;  D,  Didelphys  before  lactation;  C,  same  at 
lactation ;  E,  embryonic,  F  adult  condition  in  cow.  B  and  C  are  true  nipples,  F  a  pseudo- 
nipple  (teat). 

on  the  ventral  side  of  the  body.  In  the  marsupials  there  is  a  slight  nip- 
ple developed  from  the  bottom  of  the  pocket.  In  the  higher  groups  of 
mammals  the  first  appearance  of  the  milk  glands  is  the  formation  of  a 
'  milk  line, '  a  ridge  on  either  side  of  the  body  from  in  front  back  to  the 
inguinal  region.  This  is  soon  divided  into  *milk  points '  from  each  of 
which  there  is  an  ingrowth  of  epidermis  into  the  corium,  the  interme- 
diate parts  of  the  line  disappearing.  Each  of  the  points  may  develop 
into  a  definitive  mamma,  but  not  all  of  them  come  to  full  development, 
for  the  number  in  the  adult  is  less  then  in  the  embryo,  varying  from  a 
single  pair  to  eleven  in  Centetes,  the  number  roughly  corresponding  to 


SKELETON. 


37 


the  number  of  young  at  a  birth.  This  method  of  formation  explains 
the  varying  position  of  the  niammse  and  also  the  occasional  occurrence  of 
more  than  the  normal  number  (polymastism)  in  man  and  other  mam- 
mals. Each  gland  is  provided  with  a  nipple  and  of  these  there  are  two 
kinds  (fig.  29).  In  the  one  the  whole  surface  on  which  the  lacteal 
ducts  empty  becomes  elevated,  in  the  other  the  region  around  the 
openings  of  the  ducts  becomes  drawn  out  into  a  tube  with  the  ducts 
opening  at  the  bottom  (ungulates). 

THE  SKELETON. 

The  term  skeleton  as  used  here  is  applied  to  any  of  the  harder  parts 
of  the  body,  developed  from  the  mesoderm  and  serving  for  support, 


Fig.  30. — Diagram  of  the  skeletogenous  tissue  in  the  caudal  region  of  a  vertebrate. 
bv,  blood-vessels;  epmu,  epaxial  muscles;  hs,  horizontal  partition;  hymy,  hypaxial  muscles; 
msd,  msv,  dorsal  and  ventral  median  septa;  mys,  myosepta;  n,  spinal  cord;  nc,  notochord. 

for  the  attachment  of  muscles,  for  protection  and  the  like.  This  ex- 
cludes any  purely  epidermal  hard  parts,  and  these  have  been  included 
with  the  integument. 

As  the  skeleton  can  only  develop  where  there  is  mesenchyme,  the 
distribution  of  the  chief  skeletogenous  parts,  sometimes  called  the 
membranous  skeleton,  may  be  given  here,  continuing  the  account  from 
page   16.     First  is  the  corium,  immediately  beneath  the  epidermis, 


38         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

forming  an  envelope  around  the  internal  structures.  This  connects 
in  the  middle  line,  above  and  below,  with  a  longitudinal  partition  which 
separates  the  muscle  masses  of  the  two  sides.  This  partition  splits 
to  pass  on  either  side  of  the  central  nervous  system  and  the  notochord, 
and,  just  beneath  the  peritoneum,  around  the  viscera.  From  the 
median  partition  sheets  of  mesenchyme  (myosepta)  pass  vertically 
between  the  myotomes  to  the  dermal  layer,  they  being,  like  the  myotomes, 
metameric.  Then  there  is  a  horizontal  sheet  on  either  side  which  lies 
between  the  epaxial  and  hypaxial  muscles  (p.  127).  Not  all  parts  of 
this  membranous  skeleton  develop  hard  structures,  but  these  are  most 
apt  to  arise  at  the  intersection  of  the  various  planes. 

The  skeletal  structures  are  divided  into  the  dermal,  arising  in  the 
outer  mesenchymatous  envelope,  and  the  endoskeleton,  formed  in 
the  other  parts  and  lying  deeper  in  the  body.  The  dermal  skeleton 
includes  the  scales  of  fishes,  the  dermal  armor  of  many  reptiles  and 
fossil  amphibians  and  the  bony  scales  in  the  skin  of  crocodilians  and 
some  mammals.  In  the  strict  sense  the  so-called  membrane  bones  of 
the  skull  and  the  cleithrum  of  fishes  and  the  clavicle  and  episternum 
of  higher  vertebrates  should  be  included  here,  since  they  apparently 
have  been  derived  from  dermal  ossifications,  but  convenience  of  treat- 
ment necessitates  their  consideration  with  the  endoskeleton,  with  which 
they  are  intimately  associated.    ; 

It  is  a  question  whether  the  dermal  or  the  endoskeleton  is  the  older.  The  most 
primitive  of  the  living  species,  the  cyclostomes,  have  no  exoskeleton,  but  have 
cartilage  developed  to  some  extent.  In  development,  also,  cartilage  always  ap- 
pears before  there  is  a  trace  of  the  exoskeleton.  On  the  other  hand,  some  of  the 
oldest  fishes  known  have  a  well  developed  dermal  armor,  while  the  best  preserved 
ostracoderms  show  no  trace  of  an  internal  skeleton.  The  external  skeleton  has 
probably  arisen  as  a  means  of  protection,  the  internal  as  a  result  of  muscular  or 
other  strains. 

Bones  are  connected  (articulated)  with  each  other  in  different  ways* 
They  may  be  so  articulated  that  one  can  move  on  the  other  (diar- 
throsis)  or  there  may  be  no  motion  possible  (synarthrosis),  each  with 
several  varieties.  Of  the  immovable  joints  there  may  be  sutures, 
where  the  two  bones  are  connected  by  the  interlocking  of  saw  tooth-like 
projections,  or  the  two  may  be  united  by  bony  growth  (anchylosed) 
so  that  the  line  between  the  two  disappears.  In  those  cases  of  diar- 
throdial  joints  where  there  is  much  motion  there  is  usually  a  closed  sac, 
lined  by  a  synovial  membrane  between  the  two  bones.  This  mem- 
brane secretes  a  fluid  which  lubricates  the  surfaces. 


SKELETON. 


39 


Cartilages  and  bones  are  covered  on  their  outer  surfaces  by  an 
envelope  of  connective  tissue,  called  respectively  perichondrium  or 
periosteum.  These  membranes  form  the  means  by  which  muscles 
are  attached  to  the  bones  and  by  which  blood-vessels  obtain  entrance  to 
them.     The  periosteum  is  also  a  seat  of  bone  formation. 

DERMAL  SKELETON. 

When  present,  the  dermal  skeleton  arises  by  a  marked  prolifera- 
tion of  cells  at  definite  points  in  the  corium.  These  cells  become 
specialized  (scleroblasts,  odontoblasts  or  osteoblasts)  for  the 
deposition  of  salts  of  lime  plus  a  varying  amount  of  organic  matter 
(ossein).  Upon  limy  plates  formed  in  this  way  other  parts,  also 
calcareous,  may  be  laid  down  by  the  basal  surface  of  the  epidermis, 
so  that  the  whole  dermal  element  may  be  in  part  mesenchymatous, 
in  part  ectodermal  in  origin. 


-^^  -s^? 


Fig.  31. — Cross-sections  of  developing  scale  of  Acanthias.     c,  stratum  corneum;  J,  dentine 
of  scale;  ee,  enamel  organ;  m,  stratum  Malpighii;  />,  pulp. 

It  is  generally  thought  that  the  primitive  dermal  skeleton  resembled 
that  of  existing  sharks,  and  that  from  the  hypertrophy  or  fusion  of 
such  scales  the  so-called  membrane  bones  have  arisen.  Then  the 
scales  of  other  vertebrates  are  to  be  traced  back  to  an  elasmobranch 
ancestr)%  while  teeth  are  thought  to  be  modified  scales.  Hence  the 
structure  and  development  of  the  elasmobranch  scale  should  be 
understood. 

At  regular  intervals  in  the  skin  of  a  shark  there  is  a  multiplication  of 
cells  of  the  corium,  each  aggregation  forming  a  small  papilla  which 
projects  above  the  surrounding  corium,  carrying  with  it  the  basal  layer 
of  the  epidermis.  The  surface  cells  of  the  papilla  and  the  region 
around  it  becomes  converted  into  osteoblasts  which  secrete  calcic 


40 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


salts  on  their  outer  ends,  thus  forming  a  small  plate  of  dentine  (p.  24) 
with  a  central  spine  into  which  the  papilla  extends.  The  overlying 
epidermal  cells  form  an  enamel  organ,  the  lower  surface  of  which 
secretes  an  even  harder  layer  of  enameP  upon  the  dentine  base,  this 
being  thickest  on  the  tip  of  the  spine.  The  mesenchyme  in  the  papilla 
is  the  so-called  pulp.  With  continued  growth  the  spine  projects  through 
the  epidermis,  giving  the  skin  of  the  shark  its  characteristic  rough 
(shagreen)  condition.     This  is  the  placoid  type  of  scale. 

FISHES. — In  the  adult  elasmobranchs  the  scales  may  be  large  and  remote  from 
each  other  (skates)  or  small  and  closely  set.  In  the  torpedo  scales  are  lacking, 
while  in  the  chimaeroids  they  occur  only  on  the  claspers,  on  the  frontal  horn,  and 
as  extreme  forms,  in  a  great  spine  in  front  of  the  dorsal  fin. 


Fig.  32. — ^Ventral  armor  of  Stegocephals  (after  Credner-Zittel).     A,  Branchiosaurus;  B, 
detail  of  same;  C,  detail  of  Archegosaurus;  D,  of  Petr abates. 

A  few  ganoids  lack  scales  {Polyodon),  while  the  sturgeon  have  minute  granules 
and  five  rows  of  large  plates  along  the  sides.  Amia  has  scales  of  the  cycloid  type, 
soon  to  be  described.  With  these  exceptions  the  ganoids  have  ganoid  scales,  which 
are  rhomboid  in  outline  and  joined  to  each  other  like  parquetry.  They  consist  of 
two  layers,  the  lower  apparently  homologous  with  the  dentine  of  sharks,  except  that 
it  is  formed  in,  not  on,  the  corium.  The  outer  layer  of  ganoin  is  formed  by  the 
corium  and  consequently  cannot  be  enamel  as  once  was  thought. 

A  few  teleosts  are  scaleless  (some  eels),  but  elsewhere  scales  are  formed  in 

pockets  in  the  corium  (fig.  181).     At  first  they  lie  side  by  side,  but  with  growth  they 

overlap  like  shingles.     There  is  only  one  layer  of  bone  mixed  with  a  large  amount 

of  ossein.-    In  cycloid  scales  the  element  is  circular  and  is  marked  with  concentric 

and  radiating  lines.     The  ctenoid  scales  differ  in  having  the  posterior  edge  of 

*  There  is  some  question  whether  this  layer  is  really  enamel;  the  usual  statement  as  to 
its  nature  is  followed  here. 


SKELETON. 


41 


each  scale  truncate  and  this  edge  and  the  surface  toothed.  Cycloid  and  ctenoid 
scales  intergrade  and  both  kinds  may  occur  on  the  same  fish  (many  gobiids). 

AMPHIBIA. — A  dermal  skdeton  occurs  in  the  recent  amphibians  only  as  rows 
of  plates  in  the  cutaneous  rings  on  the  bodies  of  the  caecilians  and  in  the  skin  of  the 
back  of  a  few  exotic  toads.  In  some  fossil  stegocephalans  there  was  a  ventral 
armor  and  in  others  one  protecting  the  whole  body.  The  ventral  exoskeleton, 
sometimes  of  scales  or  plates,  sometimes  long  bars,  is  arranged  in  oblique  rows, 
and  is  interesting  as  probably  being  the  source  of  the  gastralia  found  in  many 
reptiles  {infra).  Epistemum  and  clavicle  were  possibly  dermal  in  these  forms, 
but  they  will  be  described  in  connection  with  the  shoulder  girdle.  Apparently 
certain  of  the  gastralia  of  these  fossils  were  modified  into  comb-like  organs  which 
have  been  thought  to  have  sexual  significance. 

REPTILES. — The  dermal  skeleton  is  best  developed  in  the  turtles  of  Hving 
reptiles,  though  here  it  is  closely  associated  with  the  endoskeleton.  The  dermal 
plates  form  a  box  for  the  protection  of  the  body.  This  consists  of  a  dorsal  carapace 
and  a  ventral  plastron,  united  to  varying  extents  and  each  consisting  of  a  number 
of  elements.     In  the  carapace  there  is  a  middle  line  of  neural  plates  (fused  vAih. 


F'G.  ^^. — Section  through  developing  vertebra,  rib  and  exoskeleton  of  Chelone  imbricata, 
after  Gotte.  c,  cutis;  cs,  primitive  vertebral  body,  ep,  epidermis;  m,  external  oblique  muscle; 
p,  perichondriiun;  r,  rib;  sp,  spinal  process. 


the  vertebrae),  marginal  plates  around  the  margin,  and  costal  plates,  fused  to 

the  ribs,  between  neurals  and  marginals.  The  plastron  (fig.  34)  usually  consists 
of  nine  plates,  wholly  dermal,  the  names  shown  in  the  figure.  The  three  posterior 
pairs  are  regarded  as  the  same  as  the  gastralia  of  other  reptiles,  the  anterior  pair  as 
the  clavicles,  while  the  unpaired  entoplastron  is  supposed  to  be  homologous  with 
the  epistemum  of  other  tetrapoda. 

Some  of  the  extinct  crocodilia  were  armored  with  closely  applied  scales  and 
these  have  been  retained  in  the  existing  species  in  a  reduced  condition.  They 
also  have  well  developed  gastralia.  These  are  of  rods  dermal  bone  in  the  ventral 
body  wall  between  the  true  ribs  and  the  pelvis,  and  so  closely  resemble  ribs  that 
they  were  called  'abdominal  ribs.'  They  do  not  meet  in  the  middle  line;  each, 
except  the  first,  consists  of  two  distinct  parts,  and  the  pairs  correspond  to  the 
somites  in  number.  In  Sphenodon  (fig.  35)  the  gastralia  are  more  numerous  than 
the  somites. 

In  a  few  lizards  there  are  dermal  scales,  while  the  extinct  stegosaurs  had 


42 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


dermal  ossicles,  sometimes  of  great  size  (plates  a  yard  across,  spines  half  a  yard  long) 
in  the  dorsal  region. 

BIRDS. — Recent  birds  lack  all  dermal  ossifications,  but  Archcdopteryx  had 
gastralia. 

Mammals  rarely  have  dermal  bones.  They  are  known  in  the  extinct  zeuglo- 
dont  whales  and  in  several  fossil  edentates,  but  in  the  living  species  they  occur 


Fig.  34. — ^Plastron  of  Trionyx.  en,  ento- 
plastron;  ep,  epiplastron;  hpp,  hypoplastron; 
hyp,  hyoplastron;  xp,  xiphiplastron. 


Fig.  35. — ^Ventral  ends  of  ribs  (r) 
and  gastralia  (g)  of  Sphenodon. 


only  in  the  armadillos  where  they  form  a  complete  armor  above,  the  plates 
arranged  in  transverse  rows,  some  of  which  are  movable  on  each  other.  In  the 
extinct  glyptodons  they  formed  an  inflexible  case.  It  is  uncertain  whether  these 
are  a  new  acquisition  in  the  edentates  or  have  been  inherited  from  non-mammalian 
ancestors. 

THE  ENDOSKELETON. 


The  endoskeleton  may  pass  through  three  stages  in  its  develop- 
ment, including  the  membranous  stage.  From  this  it  may  pass  through 
a  cartilage  stage  before  becoming  bone,  or  it  may  in  part  develop 
directly  into  bone  from  membrane,  or,  lastly,  it  may  never  pass  beyond 
the  cartilage  stage.  Thus  only  the  membranous  stage  is  constant. 
These  dijfferences  in  development  are  of  great  importance  in  tracing 
homologies  between  bones  in  different  groups,  but  the  distinction  be- 
tween bones  developing  directly  from  membrane  (membrane  bones) 


SKELETON.  43 

and  those  passing  through  a  cartilage  stage  (cartilage  bones)  can  only 
be  recognized  by  following  the  ontogeny  of  the  element  in  question. 

As  stated  above,  there  is  much  evidence  to  show  that  the  membrane  bones  are 
dermal  bones  which  have  sunk  to  a  deeper  position  and  have  become  secondarily 
associated  with  the  endoskeleton.  This  is  especially  evident  in  the  skulls  of  some 
of  the  lower  ganoids.  Ossification  of  cartilage  takes  place  in  two  wa]^.  In 
ectochondrostosis  the  deposit  of  lime  salts  begins  on  the  deeper  surface  of  the 
perichondrium  and  gradually  invades  the  cartilage.  In  entochondrostosis  the 
cartilage  becomes  broken  down  in  the  interior,  some  of  the  cells  becoming  modified 
into  osteoblasts,  and  from  these  as  centres  of  ossification,  the  process  of  bone  forma- 
tion extends  in  all  directions.  In  ectochondrostosis  at  least,  the  centres  of  ossifica- 
tion may  have  been  derived,  phylogenetically,  from  elements  of  the  dermal  skeleton. 

In  ossification  the  bone  is  developed  in  layers,  between  which  the  osteoblasts  are 
arranged.  In  the  elasmobranchs  the  skeleton  is  frequently  strengthened  by 
deposits  of  lime,  but  this  calcified  cartilage  differs  from  bone  in  that  the  deposits 
of  lime  take  the  form  of  polygonal  plates  and  there  are  no  lacunae. 


©  CO)  (Q 


Fig.  36. — Diagram  of  growth  of  bone.  A,  from  an  animal  recently  fed  with  madder 
causing  a  layer  of  bone  (black)  colored  by  the  dye;  B,  later,  no  madder  fed  for  some  time, 
a  deposit  of  colorless  bone  on  outside  of  colored  layer,  internal  layer  thinner;  C,  still  later, 
outer  layer  thicker,  inner  layer  absorbed. 

Many  bones  increase  in  length  by  the  addition  of  epiphyses  at  the  ends.  These 
are  separate  ossifications  which  only  unite  with  the  main  bone  at  the  time  the  adult 
condition  is  reached.  The  increase  in  diameter  has  some  interesting  features.  In 
animals  fed  with  madder,  the  bone  formed  during  the  feeding  is  colored.  In  this 
way  it  is  found  that  the  new  bone  (fig.  36,  ^)  is  laid  down  on  the  outside  of  the 
other,  and  that  with  growth  {B  and  Q,  the  'marrow  cavity'  on  the  inside  is  in- 
creased in  size  by  the  resorption  of  the  bone  already  formed. 

For  convenience  of  treatment  the  endoskeleton  is  divided  into  axial 
and  appendicular  portions,  the  axial  consisting  of  the  vertebral  column 
(backbone)  and  the  skull,  together  with  the  ribs  and  sternum  which  are 
closely  associated  with  the  vertebrae.  The  appendicular  skeleton  in- 
cludes the  framework  of  the  limbs  and  fins  and  the  girdles  to  which 
they  may  be  attached. 

Axial  Skeleton. 

Both  the  skull  and  the  vertebral  column  surround  and  protect  the 
brain  and  spinal  cord,  and  in  this  way  the  skull  is  an  enlarged  and 


44 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


specialized  portion  of  a  continuous  axis,  but  it  is  not  possible  to  carry 
the  comparison  into  details.  The  idea  of  Oken  that  the  skull  is  a  com- 
plex of  three  or  four  vertebrae  has  long  been  overthrown.  The  skull 
differs  markedly  from  the  vertebral  column  in  the  presence  of  numerous 
membrane  bones. 

Vertebral  Column. 


The  notochord  (p.  12)  is  the  foundation  around  which  the  verte- 
brae and  the  posterior  part  of  the  skull  are  developed.     It  is  a  cylin- 


FiG.  37. — Section  of  developing  vertebra  of  45  mm.  Amhlystoma.  c,  cartilage  of  inter 
centrum;  cs^,  outer  chorda  sheath;  cs^,  inner  chorda  sheath;  dm,  dura  mater;  e,  epithelioid 
layer  of  notochord  (elastica  interna) ;  end,  endorhachis,  torn  froni  wall  of  vertebral  canal; 
np,  neurapophysis  (ossified);  ns,  neural  spine  of  preceding  vertebra;  nt,  notochord;  sc, 
spinal  cord  sd,  subdmral  space. 

drical  rod  of  entodermal  origin,  without  segmentation,^  extending  from 
the  infundibulum  (see  brain)  to  the  posterior  end  of  the  body.  Its  cells 
become  vacuolated  and  at  length  most  of  the  protoplasm,  together 
with  the  nuclei,  migrate  to  the  surface  of  the  cord,  where  they  appear 
like  an  epithelium,  which,  together  with  its  basal  membrane,  is  called 
the  internal  elastic  membrane  (elastica  interna,  fig.  37,  e). 

*  Segmental  undulations  occur  in  the  notochords  of  some  mammals,  but  their  significance 
is  not  clear. 


SKELETON. 


45 


Next,  mesenchymatous  cells,  derived  from  the  sclerotomes,  form 
a  notochordal  sheath,  boynded  externally  by  an  elastica  externa. 
The  mode  of  formation  and  the  history  of  the  sheath  vary  in  different 
groups,  for  accounts  of  which  reference  must  be  made  to  special 
papers.  Other  skeletogenous  tissue  extends  outward  from  the  sheath 
toward  the  periphery,  as  described  on  a  previous  page  (p.  38,  fig.  30) 
from  which  the  ribs  of  all  vertebrates  are  developed,  the  cyclostomes 
passing  but  little  beyond  this  membranous  condition  in  the  tnmk 
region. 

With  the  appearance  of  cartilage  segmentation  is  introduced  into 
the  skeleton.     As  cartilage  is  firm  and  comparatively  unyielding,  in 


Fig.  38.  Fig.  39. 

Fig.  38. — ^Two  caudal  vertebrae  of  alligator,  c,  centrum;  ha,  haemapophysis;  hs, 
haemal  spine;  na,  neurapophysis;  ns,  neural  spine;  poz,  prz,  post-  and  prezygapophyses; 
t,  transverse  process.     The  arrow  passes  through  the  neural  arch. 

Fig,  39. — Diagrams  of  {A  and  B)  fish  vertebrae  and  (C)  vertebra  from  higher  groups. 
b,  basal  stumps;  c,  capitular  head  of  rib;  ct,  centnmi;  d,  diapophysis;/r,  fish  rib;  ha,  haemal 
arch;  na,  neural  arch;  p,  parapophysis;  r,  rib;  t,  tubercular  head. 

order  that  the  trunk  may  bend,  the  cartilage  becomes  divided  into 
separate  blocks,  which,  in  order  that  they  may  be  moved  by  the  muscles 
connected  with  them,  must  alternate  with  the  myotomes.  Hence  the 
metamerism  of  the  vertebral  column  is  the  result  of  that  of  the  muscular 
system. 

A  typical  vertebra,  whether  of  cartilage  or  bone,  consists  of  several 
parts,  the  names  of  which  are  necessary  for  the  understanding  of  the 
following  account.  Surrounding  the  notochord  is  the  body  or  centrum, 
developed  from  the  notochordal  sheath  or  from  tissue  surrounding  it. 
A  neural  arch,  enclosing  the  spinal  cord,  extends  dorsally  from  the 
centrum.     It  consists  of  a  plate  on  either  side  (neurapophysis),  the 


46 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


arch  being  completed  by  a  neural  spine  as  a  keystone.  Ventral 
to  the  centrum  is  a  similar  haemal  arch,  composed,  in  like  man- 
ner, of  haemapophyses  and  haemal  spine,  and  enclosing,  in  the 
caudal  region,  the  caudal  artery  and  vein,  farther  forward,  the 
coelom  and  viscera.  This  type  of  vertebra  is  common  in  many 
fishes,  and  in  the  tails  of  some  higher  forms.  In  the  lowest  fishes 
it  is  simplified  by  the  omission  of  parts,  while  in  the  higher  verte- 
brates other  structures  are  added.  Among  these  are  articular  proc- 
esses (zygapophyses)  on  the  anterior  and  posterior  faces  of  the  neural 
arch  (distinguished  by  position  as  pre-  and  post-zygapophyses) 
which  lock  the  successive  vertebrae  together  and  strengthen  the  column 
without  interfering  with  its  flexibility  (fig.  38). 


Fig.  40. — Diagrammatic  sagittal  sections  of  (^4)  amphicoelous;  (B),  procoelus;  (C),  opistho 
ccelous;  and  (D),  amphplatyan  vertebrae;  the  head  supposed  to  be  at  the  left. 

In  all  vertebrates  above  fishes  most  of  the  vertebras  bear  transverse 
processes  (pleurapophyses),  extending  laterally  on  either  side.  Of 
these  there  are  two  kinds,  a  parapophysis  arising  from  the  centrum, 
and  a  diapophysis  projecting  from  the  neural  arch.  The  ribs  articu- 
late with  the  ends  of  these,  as  will  be  explained  later.  The  distinctions 
are  the  most  marked  in  the  lower  vertebrates,  but  careful  comparisons 
show  them  in  the  mammals.  Other  processes,  of  less  frequent  occur- 
rence, will  be  mentioned  below  in  connection  with  the  groups  in  which 
they  occur. 

The  ends  of  the  centra,  where  they  articulate  with  each  other, 
may  take  five  different  shapes.  They  may  be  hollow  at  both  ends 
(amphicoelous);  they  may  fit  together  with  a  ball  and  socket  joint, 
the  hollow  being  sometimes  in  front  (proccelous),  sometimes  behind 
(opisthocoelous).  In  the  mammals  flat  or  amphyplatyan  conditions 
are  common,  while  in  birds  saddle-shaped  ends  occur  (figs.  40,  49). 

In  the  history  of  vertebrae  both  comparative  anatomy  and  embryology  agree 
that  the  process  of  vertebral  formation  began  with  the  arches  and  extended  thence 


SKELETON. 


47 


to  the  sheath  of  the  notochord.  In  what  must  be  considered  the  most  primitive 
condition  the  arches  extend  no  further  than  the  sheath  and  nothing  comparable  to 
a  centrum  is  found,  even  when  ossification  occurs.  In  the  formation  of  centra  two 
methods  of  extension  of  cartilage  to  the  chordal  region  are  known.  In  the  elasmo- 
branchs  immigrating  cells  from  the  arches  break  through  the  elastica  externa  and 
distribute  themselves  through  the  sheath,  converting 
it  into  cartilages.  In  other  vertebrates  (fig.  43)  the 
immigrating  cells  extend  around  the  elastica  externa 
so  that  the  sheath  eventually  comes  to  lie  inside  the 
centrum. 

In  many  fishes  and  fossil  amphibians 
another  element,  the  intercalare,  enters  into 
the  composition  of  the  neural  arch  on  either 
side.     The  intercalaria  lie  above  and  behind       „  ^     .      _.  u 

Fig.  41. — ^Trunk  vertebrae 

the  neurapophyses  and  may  expand  dorsally  of  Rhynchohatus,  after  Dum€- 
so  that  the  arch  is  completed  by  them  above.  tercakr^^te;^'r^meS; 
The  dorsal  root  of  the  spinal  nerve  usually  «,. neural  process;  r,  rib;  s, 

.  spinous  process. 

passes   through  the  mtercalare,   the   ventral 

through  the  neurapophysis,  but  both  roots  may  pass  between  them. 
Similar  intercalaria  may  occur  in  the  haemal  arclL  In  the  trunk  region 
there  may  be  separate  elements  of  the  centra;  in  each  somite  a  trans- 
verse cartilage  (hypocentnun)  across  the  under  side  of  the  neural 
sheath,  and  a  pleurocentrum  on  either  side,  behind  the  hypocentrum 
(fig.  42).       . 


Fig.  42. — Stegocephalan  vertebrae,  after  Zittel  and  Woodward.  A,  phyllospondylous; 
B,  rhachitomous  of  Chelydrosaurus;  C,  Callopterus;  D,  embolomerous  of  Eurycormus;  hs^ 
hypocentrum  arcuale;  hp,  hypocentnim  pleurale;  np,  neurapophysis;  «5,  neural  spine. 

Comparisons  of  different  adult  vertebrae  show  that  these  vertebral  elements 
may  combine  in  different  ways,  though  they  have  not  been  recognized  in  the  on- 
togeny of  the  higher  forms.  Apparently  the  phyllospondylous  vertebra  of  some 
stegocephals  (fig.  42)  are  formed  of  hypocentrum  and  neural  arch,  both  contribu- 
ting to  the  hollow  transverse  process.  In  others  haemal  arch  and  hypocentrum 
unite,  while  the  pleurocentra  meet  and  fuse  above  the  notochord.  Expansion  of 
these  makes  the  vertebral  column  look  like  a  series  of  interposed  triangles  (fig.  42  C). 


48 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


This  is  the  rhachitomous  or  temnospondylous  vertebra.  Still  farther  expansion 
of  hypo-  and  pleurocentra  causes  the  former  to  unite  with  the  neural  arch,  while 
the  two  pleurocentra  meet  below  the  notochord  (fig.  42  D),  the  result  being  two  rings 
in  each  somite,  the  embolomerous  vertebra,  which  occurs  in  some  stegocephali, 
some  fossil  ganoids  and  in  the  tail  of  Amia.  Lastly  these  two  rings  (often  called 
centrum  and  intercentrum)  may  fuse,  giving  the  typical  centrum. 

The  neural  and  haemal  spines  which  complete  the  arches  are  formed  by  seg- 
mental chondrifications  of  the  interspinous  ligament  which  runs  the  length  of  the 
body  above  and  between  the  halves  of  the  neural  arches. 

The  vertebrae  are  outlined  at  an  early  stage  of  the  embryo  and  their 
number  is   not   subsequently   increased.     Consequently   increase   in 


Fig.  43. — Earlier  and  later  stages  of  development  of  a  vertebra  of  Amhly stoma,  cc, 
cartilage  in  centre  of  vertebra;  ei,  elastica  interna;  i,  incisure  cutting  through  ic,  intercentral 
cartilage;  «,  notochord;  »5,  notochordal  sheath;  v,  vertebra  (bone)  black. 

length  of  the  vertebral  column  can  only  occur  by  growth  of  the  vertebrae 
themselves.  When  first  formed  each  centrum  encircles  the  notochord 
and  prevents  its  increase  in  diameter  at  this  point,  while  between  the 
centra  it  can  expand.  As  a  result  the  notochord  soon  resembles  a  string 
of  beads  (moniliform)  with  intervertebral  enlargements.  Then,  as 
additions  are  made  to  the  centra  to  increase  their  length,  the  new  parts 
must  form  around  the  intervertebral  enlargements  and  in  this  way  the 
ends  of  the  centra  become  cup-shaped  and  th^  amphiccelous  condition 
(fig.  43,  /)  is  produced.     In  some  urodeles  this  stage  is  followed  by 


SKELETON. 


49 


the  deposition  of  cartilage  in  the  cups  (fig.  43,  //)  producing  inter- 
vertebral constrictions  of  the  cord.  As  this  progresses  absorption  of  the 
cartilage  begins  between  the  ends  of  the  vertebrae  (ic)  and  continues 
in  such  a  way  that  the  result  is  a  ball  of  cartilage  attached  to  the  hinder 
vertebra  and  a  corresponding  cup  in  the  one  in  front;  in  other  words, 
an  opisthoccelous  condition. 

Several  regions  may  be  differentiated  in  the  vertebral  column,  these 
being  the  most  numerous  in  the  higher  groups  of  vertebrates.  These 
are  (i)  the  cervical,  in  the  neck,  with  great  reduction  or  even  absence 
of  ribs;  (2)  the  thoracic,  following  the  cervical,  with  distinct  ribs;  (3) 


Fig.  44. — Section  through  atlas 
(at)  and  axis  (ax)  of  fowl,  cut  sur- 
faces lined,  e,  epistropheus;  /,  facet 
for  articulation  with  skull;  /,  trans- 
verse ligament. 


Fig.  45. — ^Proatlas,  atlas 
and  axis  of  alligator,  a,  atlas ; 
e,  epistropheus  (axis) ;  p,  pro- 
atlas;  r,  rib  of  third  vertebra; 
ra,  re,  ribs  of  altas  and  epis- 
tropheus. 


lumbar,  without  ribs;  (4)  sacral,  including  one  or  more  vertebrae  with 
which  the  pelvis  is  connected;  (5)  caudal,  the  tail,  behind  the  sacrum. 
Sometimes  the  ribs  extend  back  to  the  sacrum  so  that  thoracic  and 
lumbar  cannot  be  distinguished,  all  being  then  grouped  as  dorsal. 
Then  in  the  fishes  and  some  higher  vertebrates  (snakes,  whales,  etc.) 
sacral  vertebrae  are  not  differentiated,  and  in  the  fishes  there  is  no  line 
between  cervicals  and  dorsals,  so  that  only  trunk  or  abdominal,  and 
caudal  regions  can  be  distinguished,  the  line  being  drawn  (fishes)  at  the 
point  where  haemal  arches  are  transformed  into  ribs. 

One  or  two  of  the  anterior  vertebrae  are  modified  in  the  higher 
(tetrapodous)  vertebrates  and  have  received  names.  The  first,  which 
immediately  adjoins  the  skull,  is  the  atlas.  It  bears  on  its  anterior  face 
an  articular  surface  which  receives  the  one  or  two  condyles  of  the  cra- 
nium. In  the  amniotes  the  second  vertebra,  the  axis  or  epistropheus  is 
also  specialized.     On  the  anterior  face  of  its  centrum  is  a  pivot  (the 


50 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


dens  or  odontoid  process)  on  which  the  atlas  turns.  Development 
shows  that  this  dens  is  the  centrum  of  the  atlas  which  has  separated 
from  its  own  verfebra  and  has  fused  to  that  of  the  axis. 

In  a  few  reptiles  and  possibly  some  mammals  a  so-called  proatlas  occurs  as  a 
plate  or  pair  of  plates  (fig.  45)  of  bone  between  the  atlas  and  the  skull,  in  the  posi- 
tion of  a  neural  arch.  It  is  not  certain  whether  this  is  the  remains  of  a  vertebra 
which  once  occupied  this  position,  or  is  a  new  formation.  Nor  has  it  been  settled 
whether  the  atlas  of  the  amphibians  is  homologous  with  that  of  mammals. 

In  cyclostomes,  fishes  and  aquatic  urodeles  the  posterior  end  of  the 
vertebral  column  is  concerned  in  the  formation  of  the  caudal  fin, 
which  presents  three  modifications.     The  most  primitive  is  the  diphy- 


FlG.  46. — Tails  of  fishes.     A,  young  Amia;  skeleton  (homocercal) ;  B,  diphycercal;  C, 
heterocercal;  D,  homocercal;  h,  hypurals;  «,  notochord;  s,  spinal  cord. 

cereal  tail  in  which  the  vertebral  column  runs  straight  to  the  end  of  the 
body,  the  fin  being  developed  symmetrically  above  and  below  it.  This 
is  found  in  the  young  of  all  fishes  and  in  the  adult  cyclostomes,  dipnoans, 
many  crossopterygians  and  urodeles.  In  the  heterocercal  tail,  which 
occurs  in  elasmobranchs  and  ganoids,  the  axis  bends  abruptly  upward 
near  the  tip,  and  while  retaining  the  caudal  fin  of  the  diphycercal  stage, 
has  a  second,  smaller  lobe  developed  below,  giving  the  whole  an  unsym- 
metrical  appearance.  In  the  homocercal  tail,  which  occurs  in  Amia 
and  all  teleosts  since  the  cretaceous,  there  is  the  same  upward  bend  to 
the  Vertebral  column,  but  symmetry  is  restored  externally  by  the  re- 
duction of  the  neural  arches  and  the  development  and  fusion  of  the 
haemals  into  larger  plates  (hypurals),  while  the  lower  lobe  of  the  tail 
grows  out  to  equal  the  other. 


SKELETON. 


51 


CYCLOSTOMES  have  a  persistent  notochord,  increasing  in  size  -with  the 
growth  of  the  animal,  and  lacking  constrictions  since  no  centra  are  developed. 
In  the  myxinoids  there  are  neurapophyses  and  intercalaria  deYcloped  in  the  caudal 
region;  in  the  lampreys  they  occur  in  the  trunk  as  well. 

FISHES. — In  the  elasmobranchs  the  typical  vertebrae  are  developed  in  cartilage, 
with  intercalaria  in  connection  with  the  arches.  Usually  the  centra  undergo  more 
or  less  calcification  (p.  43),  the  lime  being  either  deposited  in  concentric  rings 
around  the  notochord  (cyclospondylous  vertebrae)  or  in  radiating  plates  (astero- 
spondylous) .  In  the  trunk  region  each  centrum  often  bears  a  pair  of  transverse 
processes  with  short  ribs  at  their  extremities.  In  a  few  forms  (skates,  etc.) 
embolomerism  (p.  48)  occurs  in  the  tail,  and  in  the  holocephali  the  centra  are 
replaced  by  numerous  rings  of  cartilage.  In  skates  and  in  Chimcera  there  is  a  true 
joint  between  the  skull  and  the  column,  but  in  the  sharks  the  anterior  vertebrae  are 
fused  together  and  to  the  skull. 


Fig.  47. 
Fig.  47. — Diagrammatic  sections  of   elasmobranch  vertebrae 

C,  asterospondylous. 
Fig.  48. — Cross-section  of  teleost  vertebra;  bone,  black;  cartilage,  dotted 


Fig.  48. 
A,  B,   cyclospondylous; 


The  ganoids  vary  greatly  in  vertebral  characters,  some  of  the  Chondrostei 
having  only  cartilage  and  some  of  the  fossil  forms  lacked  centra.  On  the  other 
hand,  nearly  the  whole  vertebra  is  ossified  in  Amia  and Lepidosteus,  the  latter  having 
opisthoccele  vertebrae,  a  condition  not  reappearing  until  the  amphibians,  as  all 
other  fishes  in  which  centra  are  developed  have  amphicoelous  vertebrae. 

As  the  name  implies,  ossification  of  vertebrae  and  other  parts  is  common  in 
teleosts.  The  arches  are  almost  always  ossified,  while  the  centra  may  be,  or  those 
parts  directly  connected  with  the  arches  may  remain  cartilaginous  while  the  rest 
ossifies  (fig.  48),  so  that  the  section  presents  a  radiate  figure  as  in  the  asterospondy- 
lous sharks.  Some  teleosts  have  zygapophyses  and  a  few  genera  have  transverse 
processes  on  some  of  the  vertebrae. 

The  dipnoans,  so  far  as  ossification  of  the  vertebrae  is  concerned,  are  on  a  par 
with  the  cartilaginous  ganoids.  There  are  no  centra  and  the  notochord  grows 
throughout  life. 

AMPHIBIA,  except  the  legless  forms,  have  caudal,  sacral,  trunk,  and  a  single 
cervical  vertebra,  the  sacrals  being  single  except  in  a  family  of  extinct  anurans. 
Zygapophyses  and  both  kinds  of  transverse  processes  may  be  present. 


52 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  stegocephals  had  the  greatest  range  of  vertebral  structure,  rhachitomous, 
embolomerous,  and  amphicoelous  types  occurring,  the  first  two  even  in  the  same 
individual.  Phyllospondylous  vertebrae  (fig.  42)  are  found  only  in  the  fossil 
Branchiosauridae. 

The  cascilians  have  a  very  large  number  (up  to  275)  of  amphicoelous  vertebrae 
in  correlation  vdth  the  snake-like  body  form.  The  perennibranchs,  derotremes  and 
many  salamandrina  are  amphicoelous;  the  rest  of  the  urodeles  are  opisthocoelous. 

The  anura,  as  a  rule,  have  procoelous  vertebrae,  but  a  few  genera  have  them 
opisthoccele.  All  recent  species  have  eight  presacral  vertebrae,  but  there  were  nine 
in  the  tertiary  forms.  A  striking  feature  is  the 
fusion,  in  the  adult,  of  all  of  the  caudal  verte- 
brae into  the  well-known  rod,  the  coccyx  or 
urostyle. 

REPTILES  always  have  the  vertebrae  ossi- 
fied, although  remnants  of  the  notochord  may 
persist  in  the  centra,  of  which  all  types,  amphi-, 
pro-,  opisthocoelous  and  flat  occur  in  the  group. 
In  lizards,  snakes  and  dinosaurs  the  articulation 
between  the  successive  vertebrae  is  strengthened 
by  zygantra  and  zygosphenes,  a  cavity  on  one 
vertebra  into  which  a  projection  from  the  next 


Fig.  49.  Fig.  50. 

Fig.  49. — Cervical  vertebra  of  a  bird  showing  the  saddle-shaped  articular  surface  {af) 
on  the  centrum,  c;  cr,  cervical  rib;  nc,  neural  canal;  ns,  neural  spine;  poz,  prz,  post-  and 
prezygapophyses. 

Fig.  50. — Central  view  of  synsacrum  and  pelvis  of  hawk  (Buteo).  il,  ilium;  m, 
ischium;  p,  pubis;  pp,  pectineal  process;  s,  sacral  ribs. 


fits.  In  the  existing  species  there  are  never  more  than  two  sacral  vertebrae,  but 
the  pterosaurs  had  from  three  to  seven,  while  in  the  dinosaurs  there  might  be  ten, 
all  being  co-ossified  when  there  were  more  than  three. 

Little  is  known  of  the  theriomorph  backbone,  except  that  some  had  persistent 
notochords,  others  amphicoelous  centra.  In  the  plesiosaurs  they  were  flat,  while 
in  the  turtles  the  dorsals  are  fused  and  the  neural  spines  are  united  with  the  neural 
plates  (p.  41).  The  other  centra  vary.  Those  of  the  rhynchocephals  and  most 
dinosaurs  are  flat,  while  snakes  and  lizards,  except  the  geckos  have  them  procoelous. 
In  the  earliest  crocodiled  they  were  amphicoelous,  while  later  they  are  procoelous  or 
flat,  and  in  the  pterodactyls  they  are  procoelous  in  front,  amphicoelous  in  the  tail. 

BIRDS  usually  have  saddle-shaped  ends  to  the  centra  (the  atlas  procoelous). 


SKELETON. 


53 


Several  of  the  dorsals  are  usually  fused  for  strength,  but  the  first  presacral  is  free. 
A  characteristic  feature  is  the  sjrnsacrum,  foreshadowed  in  the  dinosaurs.  As  the 
bird  stands  on  two  feet  and  holds  the  body  obliquely,  several  of  the  dorsal  and  caudal 
vertebrae  (up  to  20)  fuse  with  the  sacrals  into  a  common  mass,  a  large  proportion 
also  uniting  with  the  pelvis.  The  true  sacrals  (three  in  ostriches,  two  elsewhere)  lie 
just  behind  the  pits  occupied  by  the  kidneys  and  may  be  recognized  by  their  lower 
articulation  to  the  pelvis.  A  few  of  the  caudals  behind  the  synsacrum  are  free,  as 
all  were  in  ArchcBopteryx,  but  the  others  in  recent  birds  are  united  into  an  upturned 
bone,  the  pygostyle. 

MAMMALS,  except  whales  where  the  sacrum  is  lacking,  have  all  the  five  verte- 
bral regions  differentiated.  With  four  exceptions  the  cervicals  are  seven  in  number 
(Manatus  australis  and  Choloepus  hofmanni,  six;  Brady  pus  torquotus,  eight;  B. 
tridactylus,  nine).  The  dorsals  (thoracics  plus  lumbars)  vary  between  fourteen  in 
armadillos  and  thirty  in  Hyrax,  but  usually  are  nineteen  or  twenty,  the  number 
of  thoracics  usually  increasing  at  the  expense  of  the  lumbars.  There  are  primi- 
tively two  sacrals,  but  others  may  unite  until  they  amount  to  nine  or  ten  in  some 
edentates.  Usually  the  centra  are  amphiplatyan,  but  in  the  cervicals  of  ungulates 
opisthocoele  vertebrae  are  common.  It  is  to  be  noted  that  the  'transverse  proc- 
esses '  of  the  cervical  vertebrae  are,  as  in  birds,  composed  in  part  of  reduced  ribs,  as 
will  be  shown  below. 

Ribs. 

Two  different  structures  are  included  under  the  common  name  of 
rib,  both  connected  at  one  end  with  a  vertebra,  the  other  supporting  the 
body  walls  around  the  viscera.  In  following  forward  the  haemal  arches 
in  the  skeleton  of  a  bony  fish  (fig.  39,^4,  B)  it  is  seen  that  when  the 


Fig.  51. — ^Vertebrae  and  ribs  of  (/)  anterior  and  (//)  posterior  trunk  region  of  Polypterus, 
after  Gegenbaur.    p,  pleiural  rib;  h,  haemapophysial  rib. 

body  cavity  is  reached  the  arch  becomes  incomplete  below,  the 
two  hasmapophyses  separating  and  coming  to  lie  just  beneath  the 
peritoneum  in  the  walls  of  the  coelom.  Above,  it  is  either  united 
directly  to  the  centrum  or  is  jointed  to  a  small  process  of  it.  More 
careful  study  shows  that  this  fish  rib  (haemapophysial  rib)  lies  in 
the  intersection  of  a  myoseptum  with  the  median  partition  of  the 
skeletogenous  tissue  (p.  38)  and  is  medial  to  the  hypaxial  muscles. 
In  the  higher  vertebrates  the  rib  is  formed  in   the  intersection,  of 


54         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  myosepta  with  the  horizontal  plate,  and  thus  is  lateral  to  the 
hypaxial  muscles  and  between  them  and  the  epaxial  series.  This 
is  the  true  or  pleural  rib.  Any  vertebra  may  bear  ribs  of  either 
kind  (including  haemal  arches)  and  the  two  kinds  frequently  coexist  on 
the  same  vertebra  in  the  trunk  of  salmonids,  clupeids  and  Polypterus, 
and  in  the  caudal  region  of  urodeles  and  some  reptiles.  Their  possible 
occurrence  in  all  parts  of  the  body  is  explained  by  the  existence  of  the 
myosepta  and  other  skeletogenous  structures  in  all  regions. 

The  haemapophysial  ribs  end  freely  below,  never  being  connected  with  a  sternum. 
In  some  aberrant  fishes  they  are  lacking,  while  in  the  ostariophysi  they  play  a  part 
in  the  'Weberian  apparatus'  connecting  the  swim  bladder  with  the  ear  (see  ear). 
The  teleosts  have,  in  addition,  numerous  rib-like  structures  which  are  not  preformed 
in  cartilage  (epineurals,  epimerals,  epipleurals)  which  are  formed  in  the  epaxial 
or  hypaxial  regions  or  in  the  horizontal  partition. 


Fig.  52. — ^Front  and  side  views  of  cervical  vertebra  of  fowl,  showing  the  cervical  rib. 
c,  centrum;  cs,  spinal  canal;  d,  diapophysis;  p,  parapophysis;  r,  rib;  va,  vertebrarterial 
canal;  the  arrow  in  the  side  view  passes  through  the  canal. 

The  typical  rib  (it  is  not  certain  whether  this  is  the  primitive  form) 
has  two  heads  for  articulation  with  the  vertebra,  a  capitular  head 
connecting  with  the  parapophysis,  a  tubercular  head  joining  the 
diapophysis.  Between  the  two  heads  and  the  centrum  is  a  space,  the 
vertebrarterial  canal,  through  which  the  vertebral  artery  passes 
(fig.  39,  C.)  The  true  ribs,  which  are  preformed  in  cartilage,  have 
various  extents  in  the  different  regions  of  the  body.  In  the  thoracic 
region,  where  they  have  the  greatest  extension,  the  ribs  have  to  allow 
for  changes  in  size  of  the  contained  cavity,  and  hence  parts  of  them 
are  frequently  left  unossified,  or  at  least  they  are  jointed,  the  two  parts 
being  called  vertebral  and  sternal  ribs. 

In  the  cervical  region  the  true  ribs  are  usually  greatly  reduced  and  are  lacking 
in  the  turtles.  In  many  reptiles  they  clearly  show  their  nature,  being  short, 
bicipital  and  with  their  heads  articulated  to  dia-  and  parapophyses  (fig.  45).  In 
the  birds  they  may  be  recognized  (fig.  52),  their  distal  ends  being  bent  inward  to 


SKELETON.  55 

protect  the  carotid  arteries.  In  the  mammals  they  form  the  distal  part  of  the 
'transverse  process'  of  human  anatomy,  the  vertebrarterial  canal  and  the  develop- 
ment revealing  their  true  nature. 

The  dorsal  ribs  are  very  short  in  amphibians,  never  extending  far 
from  the  backbone.  They  are  bicipital  in  most  forms,  except  the 
anura  where  they  form  small  projections  on  the  ends  of  the  transverse 


Fig.  53. — Skeleton  of  trunk  of  common  goose,  Anser  domesticus.  c,  cuneiform;  ca, 
carina;  co,  coracoid;/,  furcula  (clavicle) ; /e,  femur;  h,  humerus;  il,  ilium;  is,  ischium ;  mc, 
metacarpals;  p,  pubis;  ph,  phalanges;  r,  radius;  s,  scaphoid;  sc,  scapula;  sr,  sternal  rib;  st, 
sternum;  u,  uncinate  process;  ul,  ulna;  vr,  vertebral  rib;  2,  3,  4,  digits. 

processes.  In  the  amphibia  the  vertebral  artery  is  ventral  to  the  par- 
apophysis.  In  all  other  vertebrates  with  a  sternum  at  least  a  part  of  the 
dorsal  ribs  reach  that  structure,  encircling  the  viscera  like  the  hoops  of  a 
barrel.  Those  ribs  which  do  not  reach  the  sternum  are  called  false 
ribs.  In  most  reptiles  and  some  birds  most  of  the  thoracic  ribs  bear  an 
uncinate  process  directed  upward  and  backward  (fig.  53),  overlapping 


56         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  rib  behind  and  strengthening  the  thorax.  In  the  chelonia  the  ribs 
are  confined  to  the  dorsal  side  of  the  body  and  are  fused  to  the  costal 
plates  (dermal  skeleton)  to  form  the  carapace.  Single-  and  double- 
headed  ribs  often  occur  in  the  same  individual  of  various  groups,  and 
in  the  mammals  the  capitular  head,  instead  of  articulating  with  a 
distinct  parapophysis,  may  rest  in  a  socket  formed  by  two  successive 
vertebrae. 


Fig.  54. — Sacral  vertebrae,  ribs  and  pelvis  of  Trionyx,  obliquely  from  below.    /,  head  and 
rochanter  of  femur;  il,  ilium;  is,  ischium;  p,  pubis;  sr,  sacral  ribs;  sv,  sacral  vertebrae. 

The  pelvis  is  never  directly  united  to  the  sacrum,  but  sacral  ribs 
intervene.  These  are  distinct  in  the  reptiles  (fig.  54),  but  are  fused  to 
the  transverse  processes  in  other  groups. 

The  Sternum  (Breastbone). 

The  sternum  includes  the  skeletal  parts  on  the  ventral  side  of  the 
body,  which  are  closely  connected  with  the  shoulder  girdle  and,  except 
in  the  amphibia,  with  the  ribs.  The  fact  that  it  occurs  only  in  verte- 
brates with  legs  (it  is  lacking  in  snakes  and  caecilians)  shows  that  it  has 
arisen  in  adaptation  to  terrestrial  locomotion.  In  man  it  consists  of 
three  parts,  a  manubrium  in  front,  a  middle  piece  (gladiolus)  >  and  a 
xiphoid  (ensiform)  process  behind,  and  these  terms  have  been  car- 
ried into  other  groups. 

In  development  the  sternum  arises  in  mammals  by  the  formation  of  a  longi- 
tudinal bar  of  cartilage  in  the  linea  alba  on  either  side,  ventral  (medial)  to  the  ends 


SKELETON. 


57 


of  the  ribs,  eventually  connecting  them  together  (fig.  55).  With  continued  growth 
these  bars  of  the  two  sides  meet  atid  fuse  in  the  median  line,  forming  a  median  plate, 
the  sternum.  Later  this  separates  from  the  ribs,  and  with  the  appearance  of  bone, 
becomes  a  series  of  separate  elements,  the  sternebrae  (fig.  57),  alternating  with  the 
ribs;  by  fusion  of  sternebrse  the  parts  in  man  arise. 

In  the  amphibia  the  short  ribs  never  extend  to  the  sternum,  but  skeletal  parts 
occur  near  the  mid-ventral  line  in  a  few  forms,  which  may  be  ventral  ribs  as  they 
participate  in  the  formation  of  the  sternum.  Nothing  is  known  of  a  true  sternum 
in  the  stegocephals.  In  the  urodeles  it  is  a  short  cartilaginous  plate,  lying  mostly 
behind  the  girdle,  with  its  sides  grooved  to  receive  the  medial  ends  of  the  coracoids. 


Fig.  55. 


-Development  of  sternum  in  30  mm,  human  embryo,  after  Ruge.     cl,  lower  end 
of  clavicle;  r,  ribs;  s,  two  halves  of  stemimi;  ss,  suprastemalia. 


In  the  toads  and  their  allies  (arcifera)  it  has  hardly  passed  beyond  the  urodele 
condition,  but  the  hinder  angles  are  produced  into  long  processes.  In  the  frogs 
(firmisternia)  it  consists  of  a  slender  thread  between  the  medial  ends  of  the  girdles 
(epicoracoids),  but  in  front  it  expands  into  an  omostemum,  ossified  behind;  while 
behind  the  girdle  it  forms  a  broad  xiphistemiun,  the  anterior  part  of  which  is  bone. 

In  the  lizards  the  sternum  is  a  large  rhomboid  plate,  largely  cartilag- 
inous, sometimes  perforated  with  two  foramina  and  joined  by  a  vary- 
ing number  of  ribs  (fig.  56).  In  the  crocodilia  there  is  an  anterior 
rhombic  plate,  joined  by  two  pairs  of  ribs  and  followed  by  a  second, 
so-called  abdominal  sternum,  connected  with  from  five  to  seven  pairs 
of  ribs.  Ichthyosaurs,  plesiosaurs  and  snakes  have  no  sternum, 
while  it  was  imperfectly  ossified  in  theriomorphs  and  dinosaurs. 

In  the  birds  (fig.  53)  the  sternum  is  ossified  and  at  most  is  con- 
nected with  eight  pairs  of  ribs.     Behind  it  may  be  rounded,  perforated. 


S8 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


notched,  or  prolonged  into  one  or  two  long  processes.  In  the  ostriches 
the  ventral  surface  is  smooth  and  this  was  formerly  used  as  a  character 
separating  these  birds  as  a  group  of  ratites,  in  contrast  to  all  other 
birds  (carinatae)  which  either  use  their  wings  in  flight  or  in  swimming 
(penguins)  and  in  which  there  is  a  necessity  for  strong  wing  muscles. 
For  the  attachment  of  these  the  ventral  surface  of  the  sternum  is  de- 
veloped into  a  strong  projecting  keel  (carina).  It  is  to  be  noted  that 
a  similar  keel  is  developed  in  the  bats  and  pterodactyls. 


•  Fig.  56. — Sternum,  etc.,  of 
Iguana  tuber culata,  after  B  Ian- 
chard,  c,  coracoid;c/,  clavicle; 
e,  epistemum;  h,  humerus;  pc, 
procoracoid;  x,  xiphisternum. 


Fig.  57. — Sternum  of  guinea 
pig.  sr,  sternal  rib;  st,  sterne- 
brae;  vr,  vertebral  rib,  x,  xiphi- 
sternum. 


In  the  mammals  the  number  of  ribs  connected  with  the  sternum 
is  greater  than  in  the  lower  classes.  The  sternebrae  may  remain  dis- 
tinct throughout  life  (fig.  57)  or,  as  in  man,  they  may  fuse  into  fewer 
elements,  the  xiphoid  process  being  unconnected  with  the  ribs.  In  the 
edentates  and  rodents  elements  known  as  ossa  suprasternalia  and  pro- 
sternum  occasionally  occur  in  front  of  the  sternum,  the  significance  of 
which  is  unknown.  It  is  possible  that  traces  of  them  persist  in  the 
higher  orders,  even  in  man  (fig.  55). 


SKELETON. 


59 


Episternum  (Interciavicle). 


In  stegocephals  and  the  oldest  rhynchocephals  there  is  a  median 
bone  on  the  ventral  surface,  called  the  episternum  (fig.  58).  It 
is  rhomboid  in  front  and  may  have  a  long  posterior  process,  the  medial 
ends  of  the  clavicles  lying  ventral  to  the  broad  anterior  end.  This 
element  is  regarded  as  homologous  with  a  T-shaped  membrane  bone 
which  occupies  a  similar  position  in  lizards  (fig.  56)  and  crocodilians, 
where  it  acts  as  a  brace  between  the  shoulders.     It  arises  by  two  centres 


Fig.  58. —  Shoulder  girdles  of  (A)  Melanerpeton  and  (B)  diagram  of  Branchiosaurus,  after 
Credner,  the  determination  of  elements  after  Woodward,  cl,  clavicle;  co^  coracoid;  e, 
episternum;  s,  scapula. 

of  ossification  in  membrane  and  hence  cannot  be  the  same  as  the  su- 
prasternalia  of  mammals.  An  episternum  also  occurs  in  theriomorphs, 
pythonomorphs,  ichthyosaurs,  and  plesiosaurs,  and  possibly  the 
entoplastron  of  the  chelonians  (fig.  34,  p.  42)  is  the  same  structure. 
It  has  not  been  recognized  in  birds,  but  it  reappears  in  the  monotremes 
among  mammals  (fig.  113),  with  nearly  the  same  relations  as  in  the 
lacertilians. 


The  Skull. 

The  skull  is  a  complex  structure  and  the  last  word  concerning  its 
composition  has  yet  to  be  said.  A  century  ago  Oken  pointed  out  that 
a  series  of  parts  could  be  distinguished  in  the  mammalian  skull,  each 
of  which  somewhat  resemble  a  vertebra  in  its  general  relations,  and 
thus  laid  a  foundation  for  a  *  vertebral  theory  of  the  skull '  which  was 
farther  developed  by  Owen.  Huxley  showed  that  these  were  superficial 
resemblances,  that  the  three  or  four  vertebrae  they  would  recognize  were 
nothing  of  the  sort,  and  that  the  skull  shows  no  real  metamerism 
except  in  the  occipital  region  and  in  the  visceral  arches. 

In  its  development  the  skull,  like  the  rest  of  the  skeleton,  passes 
through  two,  and  in  the  bony  vertebrates,  three  stages:  membranous, 


6o         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

cartilaginous  and  osseous,  and  in  the  early  stages  there  is  no  trace  of  seg- 
mentation or  of  vertebrae,  the  Okenian  segments  only  appearing  with 
the  appearance  of  bone.  The  skull  may  be  divided  into  two  portions, 
a  cranium,  composed  of  a  case  for  the  brain,  and  sense  capsules  en- 
closing the  organs  of  special  sense  (ears,  eyes  and  nose) ;  and  a  visceral 
skeleton,  more  or  less  intimately  related  to  the  anterior  end  of  the 
digestive  tract. 

Development  of  the  Skull. 

Little  is  known  in  detail  of  the  development  of  the  membranous 
skull  save  that  it  envelops  the  brain  and  sense  organs,  extends  into 
the  visceral  region,  and  that  it  affords  the  substance  in  which  the  second, 
or  cartilaginous,  skull  is  formed. 


Fig.  59. — Early  chondrocranium  of  Acanthias,  after  Sewertzoff.  (The  brain  in  outline.) 
als,  alisphenoid  cartilage;  ch,  anterior  end  of  notochord;  h,  hyoid  arch;  ma,  mandibular 
arch,  not  yet  divided  into  pterygoquadrate  and  Meckelian;  oc,  otic  capsule;  /,  trabecula; 
1-5,  branchial  arches. 

The  cartilaginous  envelope  of  the  brain  and  sense  organs  is  called 
the  chondrocranium.  The  notochord  extends  forward  beneath 
the  brain  as  far  as  the  infundibulum  and  a  horizontal  cartilage  plate 
forms  on  either  side  of  it.  These  parachordal  plates  extend  later- 
ally as  far  as  the  ears,  forward  as  far  as  the  end  of  the  notochord  and 
back  to  the  exit  of  the  tenth  nerve.  A  little  later  a  cartilaginous  otic 
capsule  forms  around  each  ear  and  joins  the  parachordals,  thus  form- 
ing a  trough  in  which  the  posterior  part  of  the  brain  lies,  its  floor  formed 
of  parachordals  and  notochord  (basilar  plate)  and  its  sides  of  the 
sense  capsules. 

From  this  posterior  part  two  cartilages  extend  forward  on  either 
side,  forming  a  somewhat  similar  trough  for  the  anterior  part  of  the 


SKELETON. 


6i 


brain;  the  lower  of  these,  the  trabeculae  cranii,  join  the  anterior 
margin  of  the  basal  plate  while  the  dorsal  bars,  the  alae  temporales 
or  alisphenoid  cartilages  are  eventually  connected  with  the  anterior 
wall  of  the  otic  capsules.  In  most  vertebrates  the  trabeculae  and 
alisphenoids  develop  as  a  continuum,  but  in  some  elasmobranchs  they 
are  at  first  distinct  (fig.  59).  The  two 
trabeculae  unite  in  front  to  form  a 
median  ethmoid  plate  beneath  the 
olfactory  lobes,  beyond  which  they 
diverge  as  two  horns,  the  comua  tra- 
beculae, ventral  to  the  nasal  organs. 
The  floor  of  the  trough  is  formed  by 
the  ethmoid  plate  in  front,  while  behind 
it  is  usually  of  membrane,  but  in  the 
elasmobranchs  cartilage  gradually  ex- 
tends from  one  trabecula  to  the  other, 
closing  last  below  the  infundibulum 
and  hypophysis,  these  lying  for  a  time 
in  an  opening  (fenestra,  later  fossa 
hypophyseos),  and  after  the  closure, 
in  a  pocket  in  the  floor  of  the  chon- 
drocranium,  one  of  the  cranial  land- 
marks, the  sella  turcica. 


Fig.  60. — Early  (platybasic)  chon- 
drocranimn  of  an  elasmobranch, 
straightened  out  Compare  with  fig. 
59.  als,  alisphenoid;  ctr,  comua  tra- 
beculae; ep,  ethmoid  plate;  fhyp  fenes- 
tra h)rpophyseos;  oc,  otic  capsule;  ov, 
occipital  vertebrae;  n,  notochord;  pc, 
parachordal  plate;  tr,  trabeculae. 


In  the  elasmobranchs  and  amphibians 
the  trabeculae  are  widely  separated  until  they 
reach  the  ethmoid  plate,  a  condition  correla- 
ted with  the  anterior  extension  of  the  brain. 
This  is  the  platybasic  chondrocranium.  In 
the  other  classes  the  brain  does  not  extend 
so  far  forward  and  the  two  trabeculae  meet  just  in  front  of  the  hypophysis  (fig. 
62)  to  continue  forward  as  a  trabecula  communis  to  the  ethmoid  region.  The 
trabecula  communis  is  usually  compressed  between  the  eyes  to  a  vertical  interor- 
bital  septum.     This  represents  the  tropibasic  chondrocranium. 

In  the  more  primitive  vertebrates  the  trough  is  converted  into  a 
tube  around  the  brain  by  the  extension  of  cartilages  between  the  ali- 
sphenoid cartilages  and  the  otic  capsules  of  the  two  sides  dorsal  to  the 
brain.  This  roof  or  tegmen  cranii  is  usually  incomplete,  having  one 
or  more  gaps  or  fontanelles,  closed  only  by  membrane.  In  the  higher 
vertebrates  the  cartilage  roof  is  at  most  restricted  to  a  mere  arch,  the 
synotic  tectiun,  between  the  otic  capsules  of  the  two  sides.     Later 


62 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


a  pair  of  nasal  capsules  develop  around  the  olfactory  organs.  These 
are  usually  fenestrated  and  become  united  to  the  cornua,  alisphenoids, 
and  ethmoid  plate.     In  a  similar  way  a  sclera  (sclerotic  coat)  forms 


Fig.  6i  . — Diagram  of  early  elasmobranch  chondrocranium  in  side  view,  the  brain  out- 
lined behind,  al,  alisphenoid  plate;  bp,  basal  plate;  gc,  gill  clefts;  h,  hyoid;  hm,  hyomandib- 
ular;  I,  upper  labials;  II,  lower  labials;  nc,  nasal  capsule;  oc,  otic  capsule;  ov,  occipital 
vertebrae;  ptgq,  pterygoquadrate;  si,  suspensory  liganents;  sp,  spiracle;  tr,  trabeculae;  v, 
vertebrae;  7-F/7,  visceral  arches;  1-5  branchial  arches. 

around  each  eye,  but  since  the  eye  must  move,  this  sense  capsule  never 
unites  with  the  rest  of  the  cranium.  Behind  the  otic  capsules  a  vary- 
ing number  of  (four  in  some  sharks  and  most  teleosts,  in  others  three, 


ft..  \     '  .     .  . 

Fig.  62 . — ^\' antral  view  of  (tropibasic)  cranium  of  Lacerta  agilis  after  Gaupp.  aop, 
antorbital  plate;  bpt,  basipterygoid  process;  c,  entrance  to  nasal  conch;  col,  columella; 
fh,  fenestra  hypophyseos; /^o,  post-optic  foramen;  na,  nasal  capsule;  nf,  notochord;  of, 
optic  foramen;  pa,  prominence  of  posterior  ampulla;  pt,  pterygoid;  g,  articular  process  of 
quadrate;  tc,  trabecula  communis;  tmg,  taenia  marginalis;  tr,  trabecula;  VII,  XII 
seventh  and  twelfth  nerves. 


in  amphibia  two)  occipital  vertebrae  are  developed,  which  later  fuse 
with  the  rest  of  the  chondrocranium.  They  alternate  with  myotomes 
and  nerves  in  this  region  as  do  the  vertebrae  of  the  vertebral  column. 


SKELETON.  .  63 

The  cartilaginous  visceral  skeleton  arises  in  the  pharyngeal  region 
which  is  weakened  by  the  presence  of  the  gill  clefts.  It  consists  of  a 
series  of  pairs  of  bars,  the  visceral  arches  (fig.  6i,  I-VIT),  lying  in 
the  septa  between  the  clefts,  the  bars  of  a  pair  being  connected  below  the 
pharynx.  Each  bar,  at  first,  is  a  continuous  structure,  but  to  allow  of 
changes  of  size  in  the  pharynx,  each  becomes  divided  into  separate  parts, 
while  the  arches  become  connected  in  the  mid-ventral  line  by  unpaired 
elements,  the  copulae.  The  two  anterior  arches  are  specialized  and 
have  received  special  names,  the  first  being  the  mandibular,  the  second 
the  hyoid  arch,  the  others,  in  the  region  of  the  functional  gills,  being 
called  collectively  gill  or  branchial  arches.  The  number  of  these 
last  varies  with  the  number  of  gill  clefts,  there  being  seven  in  the  primi- 
tive sharks,  a  smaller  number  in  the  higher  groups,  in  which,  with  the 
loss  of  branchial  respiration,  their  form  and  functions  may  be  altered. 
At  first  all  are  clearly  in  the  head  region,  but  by  the  unequal  growth  of 
cranium  and  pharynx  the  gill  arches  are  carried  back.  All  of  the 
arches  are  cartilaginous  at  first. 

The  mandibular  arch  lies  in  the  region  of  the  fifth  nerve,  behind  the 
mouth  and  between  it  and  the  first  visceral  cleft  or  pocket,  the  spiracle 
or  Eustachian  tube.  The  arch  is  divided  into  dorsal  and  ventral 
halves  (fig.  61,  /),  known  respectively  as  the  pterygoquadrate  (pala- 
toquadrate  ptgq),  and  Meckelian  cartilages  (w).  In  the  elasmo- 
branchs  and  chondrostei  the  pterygoquadrate  forms  the  upper  jaw, 
lying  parallel  to  and  joined  to  the  cranium  by  ligaments  or  (chimaeroids) 
by  continuous  growth.  With  the  appearance  of  bone  a  new  upper  jaw  is 
formed,  as  described  below,  and  the  pterygoquadrate  becomes  more  or 
less  reduced,  and  ossifies  as  two  or  more  bones  with  greatly  modified 
functions.  Meckel's  cartilage  is  the  lower  jaw  of  the  lower  vertebrates, 
while  in  the  higher  it  forms  the  axis  around  which  the  membrane  bones 
of  the  definitive  jaw  are  arranged. 

The  hyoid  arch  lies  between  the  spiracle  and  the  first  true  gill  cleft, 
in  the  region  of  the  seventh  nerve.  It  divides  into  an  upper  element 
the  hyomandibular  cartilage  (fig.  6i,  hm),  and  a  ventral  portion,  the 
hyoid  proper,  which  may  subdivide  into  several  parts  {infra).  In  the 
lower  elasmobranchs  the  hyomandibular  and  the  rest  of  the  hyoid  arch 
are  closely  connected,  but  in  the  higher  fishes  the  hyomandibular  be- 
comes more  separated  from  the  ventral  portion  and  tends  to  intervene 
between  the  mandibular  arch  and  the  cranium,  becoming  a  suspensor 
of  the  jaws  (fig.  63).     Still  higher  it  loses  its  suspensorial  functions, 


64 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


becomes  greatly  reduced,  and  apparently  is  subsidiary  to  the  sense  of 
hearing  (see  auditory  ossicles),  or  it  may  be  lost,  the  matter  not 
being  decided.  The  hyoid  proper  becomes  more  or  less  intimately  con- 
nected with  the  arches  behind  and  is  largely  concerned  in  affording  a 
support  for  the  tongue. 

The  branchial  arches  are  all  similar  to  each  other  in  the  lower 
vertebrates,  but  with  the  loss  of  branchial  respiration  in  the  higher 


Fig.  63. — Ventral  view  of  cranium  and  visceral  arches  of  skate  {Rata)  after  Gegenbaur. 
cp,  copula;  h,  hyoid;  hm,  hyomandibular;  la,  upper  labials;  mk,  Meckelian  cartilage; 
nc,  nasal  capsule;  pg,  pterygoquadrate;  r,  rostrum. 


groups,  they  tend  to  become  reduced,  the  reduction  beginning  behind. 
Some  may  entirely  disappear,  others  give  rise  to  the  laryngeal  cartilages 
(see  respiration)  and  the  first  may  fuse  with  the  hyoid.  The  first  arch 
is  in  the  region  of  the  ninth  nerve;  the  others  in  that  supplied  by  the 
tenth. 


SKELETON. 


6S 


The  elements  of  the  branchial  arches  have  the  names,  beginning  above,  pharyn- 
gobranchial,  epibranchial,  cefatobranchial  and  hypobranchial,  the  copulae 
being  the  basibranchials.  The  elements  of  the  hyoid  are  correspondingly,  epi-, 
cerato-,  and  hypohyal.  These  parts  lie  in  the  medial  ends  of  the  gill  septa,  medial 
to  the  aortic  arches. 

Other  cartilages,  which  seem  to  be  of  less  morphological  importance,  occur  in 
the  same  region.  Among  these  are  the  labial  cartilages  (fig.  67,  /),  usually  two 
above  and  one  below,  which  lie  in  front  (outside)  of  the  cartilages  of  the  mandibular 
arch  of  sharks,  and  in  a  modified  form  as  high  as  some  of  the  ganoids.  By  some  they 
are  regarded  as  remnants  of  visceral  arches  of  the  preoral  region.     In  the  branchial 


Fig.  64. — Branchial  arches  of  {A)  Heptanchus;  (5),  CMamydoselache;  and  (C)  Cestracion; 
A  and  C  after  Gegenbaur,  B  after  Garman.  c,  ceratobranchial;  e,  epibranchial;  h,  hyoid; 
hb,  hyobranchial;  he,  hyoid  copula;  cbr,  cardiobranchial  (posterior  copula);  p,  pharyngo- 
branchial;  1-7,  branchial  arches. 

region  of  the  elasmobranchs  a  variable  number  of  extrabranchial  cartilages  may 
occur,  small  bars  external  and  parallel  to  the  upper  and  lower  ends  of  the  gill  arches. 
The  foregoing  sketch  of  the  chondrocranium  is  based  on  conditions  in  the 
gnathostomes,  and  ignores  the  peculiarities  of  the  cyclostomes  which  are  summar- 
ized below. 

In  the  elasmobranchs  and  cyclostomes  the  skull  is  cartilaginous 
throughout  life,  or  at  most  is  calcified  cartilage,  without  sharp  division 
into  separate  elements.  In  the  higher  vertebrates  the  cartilage  is  sup- 
plemented or  almost  entirely  replaced  by  bone  which  may  be  of  the 
two  kinds,  cartilage  bone  and  membrane  bone  (p.  42),  the  distinctions 
between  which  must  constantly  be  kept  in  mind  in  tracing  homol- 
ogies in  the  different  classes.  The  membrane  bones  are  usually 
derivatives  of  the  deeper  or  dentinal  layer  of  scales  or  teeth  which  have 
fused  together  (fig.  65)  and  have  sunk  to  a  deeper  position,  coming 


66 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


into  close  connection  with  the  elements  derived  from  the  cartilage 
skull,  in  some  cases  replacing  considerable  of  it.  The  cartilage  bones 
arise  by  an  ossification  of  the  cartilage.  Even  in  the  sturgeons  the 
chondrocranium  is  complete,  the  membrane  bones  being  superficial 
and  not  intimately  connected  with  the  deeper  parts. 


,^^' 


Fig.  65. — ^Vomer  of 
2  5  mm.  A  mbly stoma  larva, 
after  Hertwig,  showing 
the  bone  developed  by  the 
fusion  of  the  bases  of 
teeth. 


The  names  of  the  bones  are  largely  based  on  the  term- 
inology of  human  anatomy.  In  many  cases  what  appears 
as  a  single  bone  in  the  human  skull  is  represented  by 
several  bones  in  the  young  and  in  the  lower  vertebrates. 
In  these  cases  the  bones  in  the  lower  forms  are  usually 
given  names  which  indicate  their  relation  to  the  human 
bones  or  to  the  part  or  region  in  which  they  occur. 
Dermal  bones  are  apparently  the  older,  phylogenetically, 
but  for  convenience  the  cartilage  bones  are  considered 
first. 


The  chondrocranium  shows  several  centres  of  ossification,  but  only 
those  giving  rise  to  distinct  bones  are  considered  here.^    The  bones  of 


Fig.  66. — Ventral  view  of  schematic  skull,  chondrocranium  dotted,  cartilage  bones 
with  lines  and  dots.  ha.s\oc,  basioccipital;  hasisph,  basisphenoid ;  als,  alisphenoid;  exoc^ 
exoccipital;  ors,  orbitosphenoid;  presph,  presphenoid;  premax,  premaxilla;  qu,qua.<iia.te; 
quju,  quadratojugal;  squamos,  squamosal;  zygom,  zygomatic;  other  names  in  full. 

the  cartilaginous  brain  case  may  be  arranged  in  four  groups,  beginning 
behind  and  called  respectively  occipitalia,  sphenoidalia  and  ethmoi- 

^  Basi-  and  presphenoid,  for  example,  arise  each  from  two  centres,  but  in  all  vertebrates 
the  resulting  bones  are  unpaired. 


SKELETON.  67 

dalia,  there  being  two  sets  of  sphenoidalia.  The  occipitalia  arise  in  the 
occipital  vertebrae  and  in  the  basilar  plate.  Of  these  there  are  four 
(figs.  66,  67) :  A  supraoccipital  above,  an  exoccipital  on  either  side, 
and  a  basioccipital  below,  the  latter  extending  forward  into  the  basilar 
plate.  These  four  form  a  ring  around  a  central  opening,  the  foramen 
magnum,  through  which  the  spinal  cord  connects  with  the  brain. 

Just  in  front  of  the  basioccipital  the  basilar  plate  ossifies  to  form  the 
basisphenoid,  which  extends  forward  to  the  sella  turcica,  and  there  is 
succeeded  by  the  presphenoid,  arising  from  the  trabeculae,  and  ex- 
tending forward  to  the  ethmoid  plate.  On  either  side  a  bone,  the 
alisphenoid)  ossifies  in  the  cartilage  of  the  same  name,  and  comes  into 
close  relation  with  the  basisphenoid.  Father  in  front  a  second  element, 
the  orbitosphenoid,  arises  in  the  alisphenoid  cartilage  and  comes  into 
relation  to  the  presphenoid.  The  alisphenoid  bone  is  just  in  front  of 
the  otic  capsule,  but  there  is  always  a  large  gap  (sphenoidal  fissure, 
foramen  lacenmi  anterior)  between  it  and  the  orbitosphenoid,  through 
which  the  third,  fourth,  and  sixth  and  the  ophthalmic  branch  of  the 
fifth  nerve  pass,  the  rest  of  the  fifth  nerve  passing  through  the  alisphe- 
noid bone.  The  optic  nerve  usually  perforates  the  orbitosphenoid, 
but  may  pass  through  a  notch  in  its  margin. 

The  ethmoid  plate  may  ossify  into  a  median  mesethmoid  bone 
bounded  on  either  side  by  an  ect ethmoid  and  in  some  there  may  be 
added  other  bones  included  among  the  'turbinal  bones.'  The  ol- 
factory nerves  pass  on  either  side  of  the  mesethmoid,  the  ectethmoids 
(below)  in  the  mammals  developing  as  perforated  plates  (cribiform 
plate). 

A  series  of  otic  or  petrosal  bones  is  developed  in  each  otic  cap- 
sule. The  most  constant  of  these  are  a  prootic  in  front,  an  opisthotic 
behind,  the  two  usually  meeting  below  (fig.  66),  and  between  them, 
above,  an  epiotic,  concerning  which  more  evidence  is  needed.  In 
the  teleosts  and  some  other  forms  the  lateral  wall  of  the  otic  capsule 
may  develop  in  addition  a  sphenotic  in  front  and  a  pterotic  behind, 
the  latter  overlying  the  horizontal  semicircular  canal  of  the  ear.  In  the 
higher  groups  the  various  otic  bones  fuse  in  the  adult  to  a  single  petro- 
sal bone,  which  is  wedged  in  between  the  lateral  parts  of  the  basi- 
occipital and  basisphenoid. 

In  the  stegocephals,  reptiles  and  birds  the  sclera  often  gives  rise  to 
a  ring  of  sclerotic  bones  (fig.  67),  which,  however,  never  unite  with 
the  other  bones  of  the  skull.     The  nasal  capsules  often  develop  a 


68 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


lateral  ethmoid  on  the  upper  wall,  and  turbinals  on  the  medial  and 
lateral  walls. 

To  place  these  bones  in  the  terms  of  human  anatomy:  the  four  occipitalia  fuse 
to  form  the  single  occipital  of  man;  the  six  sphenoidalia  similarly  unite  to  form 
the  single  sphenoid,  the  alisphenoids  forming  the  greater  wings,  the  orbitosphenoids 
the  lesser  wings,  while  the  ethmoidalia  fuse  to  the  ethmoid. 

In  all  bony  vertebrates  the  cranial  walls  are  completed  dorsally 
by  membrane  bones,  which  in  the  lower  fishes  overly  the  tegmen  cranii, 
while  in  the  higher  groups  they  replace  it,  the  cartilage  failing  to  develop 


Fig.  67. — Dorsal  view  of  schematic  skull,  the  chondrocranium  dotted,  cartilage  bones 
with  lines  and  dots,  premix,  premaxilla;  pref,  prefrontal;  postfr,  postfrontal;  postor, 
postorbital;  squamos,  squamosal;  quju,  quadra  to  jugal;  qu,  quadrate;  inp,  interparietal; 
exoc,  exoccipital;  supratem,  supra  temporal;  other  names  in  full. 

in  the  roof.  The  number  of  these  elements  varies  between  wide  limits, 
the  following  being  the  most  constant. 

Beginning  in  front  (fig.  67),  there  are,  on  either  side  of  the  median 
line  a  pair  of  nasal  bones  covering  the  olfactory  region;  a  pair  of 
f rentals  between  the  orbits;  a  pair  of  parietals  at  the  level  of  the 
otic  capsules,  between  which  there  is  frequently  a  parietal  foramen 
for  the  connexion  of  the  parietal  eye  with  the  brain;  and  an  inter- 
parietal, arising  from  paired  centres,  between  the  parietals  and  the 
supraoccipital. 

In  the  higher  vertebrates  (where  the  interparietal  frequently  fuses 


SKELETON.  69 

with  the  supraoccipital)  tjiese  are  practically  all  of  the  membrane 
bones  in  the  cranial  roof  of  the  adult.  In  the  lower  groups  there  are 
several  other  bones,  some  of  which  may  appear  in  the  development  of 
the  higher  forms.  Thus  lateral  to  each  parietal  there  may  be  a  su- 
pratemporal ;  behind  the  orbit  a  postf rental  may  articulate  with  the 
frontal,  and  lateral  to  this,  and  forming  the  rest  of  the  posterior  wall  of 
the  orbit  a  postorbital.  Occasionally  the  superior  (or  medial)  wall  of 
the  orbit  is  formed  by  one  or  more  supraorbital  bones,  which,  when 
present,  exclude  the  frontal  from  the  orbit.  The  orbit  may  be  bounded 
in  front  by  a  prefrontal  bone,  adjoining  the  antero-lateral  margin  of 
the  frontal,  and  lateral  to  this  there  is  usually  a  lacrimal  bone.  Less 
constant  are  an  intertemporal  bone  dorsal  (medial)  to  the  aUsphenoid, 
a  pair  of  postparietal  bones  between  parietals  and  interparietals  and 
a  so-called  'epiotic'  above  each  otic  capsule,  which,  since  it  is  not  a 
cartilage  bone  and  has  no  relation  to  the  true  epiotic,  is  better  called 
the  tabulare. 

In  the  ichthyopsida,  and  to  a  less  extent  in  the  sauropsida  the 
basilar  plate  and  trabeculae  may  fail  to  ossify.  In  these  cases  the  floor 
of  the  cranium  (roof  of  the  mouth)  is  formed  by  a  membrane  bone, 
the  parasphenoid,  which  lies  ventral  to  the  cartilage  in  the  sphenoid 
region.  Farther  forward,  in  the  nasal  region,  are  an  additional  pair 
of  membrane  bones,  the  vomers.  Both  vomers  and  parasphenoids 
frequently  bear  teeth  and  their  origin  by  fusion  of  the  bases  of  teeth 
is  clearly  seen  in  developing  amphibia  (fig.  65). 

Some  think  the  parasphenoid  the  homologue  of  the  mammalian  vomer,  calling 
the  vomers  of  the  non-mammals  prevomers,  their  representatives  being  sought  in 
the  'dumb-bell  bone'  of  the  monotremes.  More  evidence  is  needed  on  these 
points. 

With  the  appearance  of  bone  the  mandibular  arch  undergoes  the 
greatest  modifications  of  all  the  visceral  arches.  Its  pterygoquadrate 
half  loses  its  function  as  the  upper  jaw  and  becomes  more  closely 
connected  with  the  cranium  in  front,  its  median  portion  disappearing, 
even  as  cartilage,  and  being  replaced  by  a  pair  of  membrane  bones, 
the  palatines  (fig.  66),  which  lie  between  the  pre-  or  parasphenoid  and 
the  vomers.  The  rest  of  the  arch  ossifies  as  two  bones  on  either  side, 
an  anterior  pterygoid  and  a  posterior  quadrate,  which  now  becomes 
the  suspensor  of  the  lower  jaw.  In  the  teleosts  and  reptiles  there  are 
a  series  of  pterygoid  bones. 

A  second  arch  of  membrane  bones  develops  outside  of  the  pterygo- 


70 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


quadrate  to  form  the  functional  upper  jaw  (figs.  66,  67)  in  all  bony 
vertebrates.  In  its  fullest  development  it  consists  of  bones  on  either 
side,  beginning  behind  with  a  squamosal,  which  overlies  the  quadrate, 
and  followed  by  a  quadrate jugal,  a  zygomatic  (malar  or  jugal), 
and  a  maxillary,  which  joins  the  premaxillary,  the  latter  forming  the 


Fig.  68. — Dorsal  and  ventral  views  of  skull  of  young  Sphenodon,  after  Howes  and 
Swinnerton,  Explanation  of  letters  used  in  figvires  of  skulls  (figs.  68  to  105)  unless  other- 
wise stated,  an,  angulare;  ao,  antorbital;  ap,  antorbital  process;  ar,  articulare;  as,  ali- 
sphenoid;  h,  basale;  hh,  basibranchial;  hh,  basihyal;  ho,  basioccipital;  hs,  basisphenoid ; 
ch,  ceratobranchial;  ch,  ceratohyal;  d,  columella;  co,  coronoid;  cp,  copula;  cr,  cranial  rib; 
<r.  eth,  cribiform  plate  of  ethmoid;  d,  dentary;  de,  dermal  ethmoid;  dee,  dermal  ectethmoid; 
eb,  epibranchial;  ee,  ectethmoid;  eh,  epihyal;  enp,  entopterygoid ;  eo,  exoccipital;  ep, 
ectoptery gold ;  epo,  epiotic;  eth,  ethmoid;  ethpp;  perpendicular  plate  of  ethmoid;  es,  extra- 
scapular;  exh,  extrabranchial;/,  frontal;//*,  frontoparietal;  g,  goniale;  ^,  hyoid;^&,  hypohyal; 
hm,  hyomandibular;  hr,  hyoid  rays;  i,  incus;  if,  infratemporal  fossa;  io,  interoperculum;  ip, 
interparietal;  j,  jugal;  I,  lacrimal;  la,  labial;  md,  mandibular;  me,  mesethmoid;  mk,  Meck- 
elian;  ml,  malleus;  mm,  mentomeckelian ;  mpt,  mspt,  mesopterygoid ;  mtp,  metapterygoid ; 
mx,  maxillary;  mxp,  maxillopalatine;  mxt,  maxilloturbinal;  n,  nasal;  na,  neural  arch;  nc, 
nasal  capsule;  no,  notochord;  o,  occipital;  oc,  occipital  condyle;  00,  opisthotic;  op,  operculare; 
OS,  orbitosphenoid ;  ot,  otic  bones;  p,  parietal;  pd,  predentary;  pe,  petrosal;  pf,  postfrontal; 
pi,  palatine;  pm,  premaxillary;  po,  preoperculare;  poo,  postorbital;  pq,  pterygoquadrate; 
prf,  prefrontal;  pro,  preorbital;  prot,  prootic;  prs,  presphenoid;  ps,  parasphenoid ;  pt,  ptery- 
goid; ptc,  pterygoid  cartilage;  pto,  pterotic;  q,  quadrate;  qj,  quadra  to  jugal;  r,  rostral;  rm, 
rostrum;  sa,  suprangulare;  sbo,  suborbital;  sc,  sagittal  crest;  scl,  sclerotic;  se,  sphenethmoid; 
sf,  supra  temporal  fossa;  sh,  stylohyal;  so,  supraoccipital;  sop,  subopereulare;  sor,  supra- 
orbital; sp,  sphenoid;  spht,  sphenoturbinal;  spl,  splenial;  spo,  sphenotic;  spt,  supra  temporal; 
sq,  squamosal;  ssc,  suprascapular;  st,  stapes;  sy,  symplectic;  t,  temporal;  tr,  transversum; 
tu,  turbinal;  ty,  tympanic;  v,  vomer;  vp,  vomeropalatine. 


tip  of  the  jaw  and  meeting  its  fellow  of  the  opposite  side.     Of  these 
only  the  maxillary  and  premaxillary  bear  teeth. 

In  the  lower  vertebrates  the  roof  of  the  skull  is  continuous,  its  only 
openings  being  those   for  the  nares  and   the  orbits.     In   the  higher 


SKELETON.  7I 

groups  vacuities  or  fossae  appear  in  the  postero-lateral  parts,  these  being 
bounded  by  bars  or  arcade^  of  bone.  At  most  there  may  be  three  of 
these  fossae.  The  more  lateral  of  these,  the  infratemporal  fossa 
(fig.  68),  is  bounded  laterally  by  the  zygomatic  and  quadratojugal, 
while  on  the  inner  side  it  is  separated  from  the  supratemporal  fossa 
by  a  squamoso-postorbital  arcade.  The  posttemporal  fossa  lies 
between  parietal,  supratemporal  and  occipital  bones.  Occasionally 
only  the  infratemporal  fossa  is  present,  or,  by  disappearance  of  the  inter- 
vening arcade,  infra-  and  supratemporal  fossae  may  unite  in  a  single 
temporal  fossa.  Lastly,  by  the  breaking  down  of  the  zygomatic- 
postorbital  bar,  the  temporal  fossa  and  the  orbit  may  unite. 

One  or  another  of  these  bones  may  disappear  in  some  groups,  either  by  fusion 
or  by  complete  dropping  out.  Occasionally  they  may  obtain  different  connexions 
and  relations  as  in  the  case  of  the  quadrate  in  mammals  (see  ear  bones)  so  that  the 
homologies  are  traced  with  difficulty.  The  complexity  is  increased  by  the  fusion  of 
membrane  bones  and  cartilage  bones  and  by  the  union  of  cranial  bones  with  those 
of  the  visceral  arches. 

In  the  lower  jaw  there  are  no  such  extensive  modifications  as  in  the 
upper.  At  most  Meckel's  cartilage  gives  rise  by  ossification  to  two 
bones  in  either  half.  Behind,  at  the  articulation  of  the  jaw  with  the 
quadrate,  there  is  an  artic- 
ular bone,  while  at  the 
tip,  at  either  side  of  the 
union  (symphysis)  of  the 
two  halves  of  the  jaw,  there 

is    rarely   a   mentO-Meck-      Fig.  69. — Reconstruction  of  developing  jaw  of  5ce/e- 
,.         ,  ,^,  ^  porus,  cartilage  dotted;  letters  as  in  fig.  68. 

elian  bone.     The  rest  of 

Meckel's  cartilage  forms  an  axis  around  which  the  membrane  bones 
which  form  the  definitive  jaW  are  arranged.  These  are,  at  most,  as 
follows:  (i)  a  dentary  which  surrounds  the  Meckelian  in  front  and 
usually  bears  teeth;  (2)  a  splenial  on  the  inner  side,  behind  the 
dentary  and  frequently  bearing  teeth;  (3)  an  angulare  on  the  lower 
side,  usually  extending  back  to  the  hind  end  of  the  jaw;  (4)  a  supran- 
gulare  on  the  outer  posterior  part  of  the  jaw;  (5)  a  coronoid  on  the 
upper  side,  afi'ording  attachment  for  the  muscles  which  close  the 
jaws;  and  (6)  a  goniale  (dermarticulare)  on  the  medial  and  ven- 
tral sides  of  the  articulare,  with  which  it  usually  fuses.  This  whole 
series  is  present  in  few  vertebrates,  dentary,  splenial  and  angulare 
being  the  most  constant. 


72 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


TABLE  OF  THE  PRINCIPAL  CRANIAL  BONES. 


Chondrocranium 


Cranium 


Membrane  bones 


Maxillary   arch 


Mandibular    arch 


Visceral 
skeleton 


Hyoid  arch 


Gill  arches 


Notochord 
parachordals 

Trabeculae 

Ethmoid 
plate 


Sense    cap- 
sules 


Lateral    line 


Membrane  bones 

Pterygoquadrate 
cartilage 

Membrane 

Meckel's  cartilage 


Membrane 


Basi-,  ex-,   and  supra- 

occipitals 

Basi-  and  ali-,  pre-  and 

orbitosphenoids 

Mes-   and  ectethmoids 


f  Pro-,  epi-,  opisth-,  pter- 
Otic      \  and  sphenotics  (petro- 

[sal) 

Optic      (Sclerotics) 

^^      .  /  Lateral    ethmoid,   tur- 
Nasal  <  L.     ,  ' 

(^  bmals 


Parietals,  frontals,  na- 
sals,   pre-    and    post- 
frontals,     supra-     and 
postorbitals 
Lacrimals,  infraorbitals 

(Premaxillary,  maxilla- 
ry, zygomatic,  quad- 
ratojugal,  squamosal 
1  Pterygoid  (ect-,  ent-, 
epi-,  mesoptery  golds), 
quadrate  (incus) 
Palatines,  vomers 
J  Articulare  (malleus), 
I  mento-Meckelian 


Gill  arches 


Dentary,  splenial,  coro- 
noid,  angulare  (tym- 
panic), suprangulare, 
goniale 

Hyomandibulare  (sta- 
pes) symplectic,  inter- 
hyal,  epi-,  cerato-, 
hypo-,  and  basihyal 
(corpus,  copula)  (col- 
umella) (lesser  cornua) 
Pharyngo-,  epi-,  cerate-, 
hypo-,  basi-,  hypohyal, 
(copula,  greater  cornua) 


SKELETON. 


73 


In  the  hyoid  and  branchial  arches  ossification  occurs  to  a  greater  or 
less  extent,  the  resulting  cartilage  bones  having  the  same  names  as  the 
corresponding  cartilages.  There  are  never  any  membrane  bones  in 
this  region.  In  the  teleosts  the  hyomandibular  ossifies  as  two  bones,  a 
dorsal  hyomandibular  and  a  lower  symplectic  which  connects  with  the 
quadrate.  There  is,  however,  a  considerable  amount  of  union  between 
the  various  arches  in  the  adults  of  all  tetrapoda,  where  the  branchial 
respiration  is  lost  and  the  arches  have  to  assume  other  functions  than 
the  support  of  gills. 

The  mode  of  suspension  of  the 
jaws  varies.  In  a  few  elasmobranchs 
the  pterygoquadrate  articulates  di- 
rectly with  the  cranium  (amphistylic) ; 
in  others  it  is  suspended  by  ligament 
and  by  the  interposition  of  the 
hyomandibular  between  the  otic  cap- 
sule and  the  hinder  end  of  the  jaw 
(hyostylic) ;  while  in  all  groups  above 
the  fishes  the  pterygoquadrate  is  more 
or  less  completely  fused  with  the 
cranium  (autostylic). 

The  ear  bones  or  ossicula  audi- 
tus  are  best  treated  together  here, 
although  their  consideration  requires 
the  mention  of  structures  not  yet  de- 
scribed. The  ear  bones  occur  only 
in  the  tetrapoda;  they  present  several 
modifications  not  readily  homologized 
with  each  other,  though  they  all  have 
the  same  function  of  conveying  sound  waves  across  the  tympanum 
to  the  inner  ear.  In  all  there  is  an  opening,  the  fenestra  vestibuli 
(f.  ovale)  in  the  lateral  wall  of  the  otic  capsule,  which  is  occu- 
pied by  a  movable  bone,  the  stapes,  of  uncertain  homologies,  but 
probably  representing  the  hyomandibular  of  the  fishes,  which  otherwise 
is  lacking  in  all  tetrapoda.  This  view  is  the  more  probable  since  in 
some  vertebrates  the  stapes  is  connected  developmentally  with  the  rest 
of  the  hyoid  arch. 

In  urodeles  and  caecilians  a  slender  process  extends  from  the  quad- 
rate across  the  poorly  developed  tympanic  cavity  to  articulate  with  the 


Fig.  70. — Diagram  of  the  middle 
ear  of  a  lizard,  after  Versluys.  a, 
articulare;c,  columella;  ec,  extracolu- 
mella;  h,  hyoid;  ie,  inner  ear;  mpt, 
pterygoid  muscle;  o,  oral  cavity;  po, 
parotic  process;  pt,  pterygoid  bone; 
5,  stapes;  /,  tympanic  cavdty;  tm, 
tympanic  membrane. 


74 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


.-••■■^N 


stapes  (fig.  82).  In  the  anurans  there  is  no  connection  of  quadrate 
with  stapes,  but  there  is  a  slender  rod,  the  columella,  extending  from 
the  tympanic  membrane  to  the  stapes.  This  columella  arises  behind 
the  tympanic  cavity  but  with  growth  is  included  in  it,  so  that  in  the 
adult  it  appears  to  run  directly  through  it.  In  the  sauropsida  the 
relations  are  much  as  in  the  anura,  but  when  ossification  sets  in,  the 
columella  may  form  several  elements.  In  development  the  columella 
in  these  forms  is  directly  connected  with  the  hyoid  arch. 

In  the  mammals  there  is 
a  chain  of  three  bones  to  carry 
the  sound  waves  across  the 
tympanic  cavity.  In  the 
fenestra  vestibuli  is  the  stapes, 
which  connects  with  an  incus 
and  lastly  comes  the  malleus, 
which  has  two  long  processes, 
a  manubrium  which  is  in- 
serted in  the  tympanic  mem- 
brane, and  a  processus  an- 
terior (Folian  process)  which 
extends  into  the  petrotym- 
panic (Glaserian)  filssure 
of  the  temporal  bone.  That 
these  parts  are  not  to  be  com- 
pared to  the  columella  of  the 
sauropsida  and  anura  is 
shown  by  the  fact  that  they 
invade  the  tympanic  cavity 
from  in  front  and  that  they 
are  in  front  of  the  chorda 
tympani  nerve,  the  columella  of  the  non-mammals  lying  behind  it. 

The  homologies  of  these  parts  seem  clear.  In  development  the 
malleus  is  the  posterior  end  of  Meckel's  cartilage,  being  in  the  position  of 
the  articulare  of  lower  groups.  It  articulates  with  the  incus,  which  in 
turn  at  first  articulates  with  the  wall  of  the  otic  capsule,  as  well  as  with 
the  stapes,  and  thus  corresponds  with  the  quadrate.  The  stapes  is 
apparently  the  same  throughout  the  whole  of  the  tetrapoda.  It  is  to  be 
noted  that  many  paleontologists  deny  the  homologies  recognized  here, 
think  that  in  the  mammals  the  quadrate  has  been  lost  in  the  glenoid 


Fig.  71. — Diagram  of  ear  bones  of  embryo  pig, 
the  tympanic  cavity  laid  open,  g,  goniale;  i, 
incus;  Ij,  lower  jaw;  m,  malleus;  mk,  Meckel's 
cartilage;  mm,  manubrium  of  malleus;  s,  stapes; 
sq,  squamosal;  z,  zygomatic.  The  outlines  of  the 
zygomatic  arch  and  the  hind  end  of  the  jaw  are 
dotted. 


SKELETON. 


75 


fossa,  and  find  the  malleus  and  incus  in  the  columella.  For  this  they 
have  no  evidence  except  comparisons  with  certain  theriomorph  reptiles. 
The  literature,  which  is  extensive,  should  be  consulted  for  details. 

The  Skull  in  the  Different  Classes, 


CYCLOSTOMES  have  only  the  cartilage  skull,  and  this  can  be  homologized 
only  in  part  with  that  of  other  vertebrates;  indeed  the  skulls  of  the  two  groups  of 
cyclostomes  are  not  easily  compared.  The  peculiarities  are  in  part  due  to  the 
development  of  a  suctorial  mouth  with  its  necessary  framework.  The  chondro- 
cranium  of  the  Ammocoete  stage  of  Petromyzon 
is  readily  understood.  Parachordals,  otic  capsules 
and  trabeculae  (fig.  72)  are  normal,  but  a  pair  of 
ventral  horns  are  problematical.  Their  position 
in  front  of  and  below  the  otic  capsule  renders 
doubtful  the  interpretation  of  hyoid  or  quadrate 
sometimes  given  them. 

The  adult  Petromyzon  has  a  typical  brain 
trough,  roofed  by  a  slender  synotic  tectum  and 
fibrous  tissue  and  closed  in  front  by  the  unpaired 
nasal  capsule,  bound  to  the  rest  by  fibrous  tissue. 
The  cranium  is  continued  forward  by  a  large  plate 
(mesethmoid  ?)  lying  dorsal  to  the  mouth,  this 
part  being  roofed  by  two  'dorsal  cartilages,'  the 
anterior  articulating  with  the  annular  cartilage 
supporting  the  mouth.  A  subocular  bar  extends 
forward  from  each  otic  region  and  an  elongate 
lingual  cartilage  extends  from  the  mouth  back 
to  the  gill  region.  Several  other  elements  occur, 
the  names  and  positions  of  which  may  be  seen  from  the  figures. 

The  myxinoid  skull,  the  development  of  which  is  unknown,  is  readily  inter- 
preted so  far  as  basilar  plate,  trabeculae  and  otic  capsules  are  concerned.  The  large 
nasal  capsule  is  continued  forward  by  a  latticed  framework  for  the  naso-hypo- 
physial  canal  and  a  bar  (pterygoquadrate)  joins  the  trabecula  of  either  side  and  in 
front  is  continued  in  a  cornual  cartilage.  The  lingual  cartilage  is  enormous  (is  it 
the  lower  jaw  as  has  been  suggested?),  is  divided  into  three  segments  and  bears  a 
dental  plate  with  teeth  at  its  tip.  There  are  cartilage  axes  to  the  tentacles  around 
the  mouth. 

The  branchial  skeleton  of  the  lampreys  consists  of  a  gill  basket  of  continuous 
cartilage  with  fenestrae  for  the  gills  and  above  and  below  them  as  well.  It  cannot 
be  homologized  with  the  branchial  skeleton  of  other  vertebrates  as  it  lies  imme- 
diately beneath  the  skin  and  is  lateral  to  gill  pouches  and  aortic  arches.  It  is  more 
easily  compared  to  the  extrabranchials  (p.  65)  of  the  elasmobranchs.  The 
branchial  apparatus  of  the  m)rxinoids  is  reduced,  consisting  of  two  true  gill  arches, 
in  front  of  which  is  another  arch,  usually  interpreted  as  a  hyoid. 


nc 

72. — Early     chondro- 
of    Ammocoete    stage 

of  Petromyzon,  after  Schneider. 

h,  hyoid;  nc,  notochord;  oc,  otic 

capsule;  tr,  trabecula. 


Fig. 
cranium 


76 


COMPARATIVE  MORPHOLOGY  OF  VEGTEBRATES. 


ELASMOBRANCHS  have  a  nearly  typical  chondrocranium  which  is  never 
divided  into  separate  elements  and  is  never  ossified.  The  floor  is  complete,  the  hypo- 
physis resting  in  a  sella  turcica.  Above  there  is  an  anterior  fontanelle,  closed  by 
membrane  and  a  posterior  fontanelle  may  occur.     The  occipital  region  typically 


Fig.  73. — ^Ventral  and  lateral  views  of  the  skull  of  lamprey  {Petromyzon  marinus),  after 
Parker,  ad,  anterior  dorsal  cartilage,  hb,  branchial  basket;  gc,  gill  cleft;  Ic,  labial  carti- 
lage; Idm,  lateral  distal  mandibular;  Ig,  lingual  cartilage;  nc,  nasal  capsule;  oc,  otic  capsule; 
on,  optic  nerve;  pc,  pericardial  cartilage;  pd,  posterior  dorsal  cartilage. 


Fig.  74. — Side  view  of  cranium  of  Bdellostotna,  after  Ayers  and  Jackson,  h,  basal 
plate;  hr,  branchial  basket;  c,  cornual  cartilage;  d,  dental  plate;  h,  hyoid;  /,  lateral  labial 
cartilage;  n,  nasal  tube;  nc,  notochord;  0,  otic  capsule,  oc,  olfactory  capsule;  pq,  pterygo- 
quadrate  bar;  sp,  suprapharyngeal  plate. 


articulates  with  the  vertebral  column  by  a  pair  of  prominences,  the  occipital  con- 
dyles, but  in  most  species  this  joint  is  not  functional,  the  skull  being  immovably 
united  to  the  backbone.  In  front  the  snout  is  supported  by  rostral  cartilages, 
usually  three  in  number,  but  these  are  frequently  fused  to  a  single  mass. 


SKELETON. 


77 


The  pterygoquadrate  and  the  Meckelian  cartilages  bear  teeth  and  form  the 
functional  jaws.  Most  species  are  hyostylic  (p.  73),  the  pterygoquadrate  being 
supported  in  front  of  the  orbit  by  a  ethmopalatine  ligament  on  either  side;  behind 
by  ligament  and  by  the  hyomandibular.  The  Notidanids  are  amphistylic,  the 
hyomandibular  being  connected  with  the  rest  of  the  hyoid  and  not  acting  as  a 
suspensor  of  the  jaws,  but  the  pterygoquadrate  bears  a  strong  process  which  ar- 
ticulates with  the  postorbital  process  of  the  cranium.  A  third  condition  is  found 
in  the  holocephalans  where  the  pterygoquadrate,  free  in  the  young,  becomes  auto- 
stylic  by  fusion  with  the  cranium. 

The  variations  in  the  branchial  skeleton  (figs.  63,  64)  are  readily  reducible  to 
the  typical  conditions.     In  living  elasmobranchs  the  number  of  gill  arches  is  five, 


Fig.  7; 


-Skull  ofSquatina,  after  Gegenbaur.     h,  hyoid;  hm,  hyomandibular;  i 
cartilages;  m,  Meckel's  cartilage;  pq,  pterygoquadrate;  r,  rostrum. 


.1*,  labial 


except  in  Hexanchus  and  Chlamydoselache  (six)  and  Heptanchus  (seven).  Hyoid 
and  branchial  arches  bear  numerous  branchial  rays  which  support  the  gills  and 
the  gill  septa,  while  smaller  cartilages  on  the  inner  surface  of  each  arch  extend  into 
the  gill  strainers. 

TELEOSTOMES  show  a  wide  range  of  structure  of  skull,  yet  the  series  so  inter- 
grade  that  no  sharp  lines  can  be  drawn.  The  chondrocranium  persists  to  a  consid- 
erable extent,  and  numerous  membrane  bones  are  present,  supplementing  those  of 
cartilaginous  origin.  With  few  exceptions  cartilage  bones  (the  four  occipitals,  orbito- 
and  alisphenoids  and  prootics  are  the  most  constant)  are  developed,  while  the  inner 
wall  of  the  otic  capsule  disappears,  so  that  the  cavity  is  connected  with  that  for  the 
brain.  Even  more  characteristic  is  the  presence  of  skeletal  structures  supporting 
the  opercular  fold  that  covers  the  external  openings  of  the  gill  slits.  This  is  in  part 
of  membrane  bones,  in  piart  of  cartilage  or  cartilage  bones.  There  are  two  parts 
to  the  opercular  fold,  a  gill  cover  or  operculum  above  and  a  branchiostegal 
membrane  below.  The  latter  is  supported  by  branchiostegal  rays,  comparable 
to  the  hyoid  branchial  rays  of  the  elasmobranchs,  while  the  operculum  contains 
membrane  bones,  there  being,  at  most,  four  of  these:  a  preoperculum  in  front, 
and  behind  this  in  a  row  from  above  downward,  operculare,  suboperculum  and 
interopercultun.  The  preoperculum,  overlies  hyomandibular,  symplectic  and 
quadrate,  and  it  is  possible  that  the  opercular  bones  have  been  developed  in  con- 


78 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


nexion  with  the  hyomandibular  rays  of  the  elasmobranchs.  There  are  five 
branchial  arches,  the  last  more  or  less  reduced.  Often  they  bear  teeth  on  their 
inner  surfaces,  thus  acting  as  accessory  chewing  organs. 


Fig.  76. — Side  view  of  skull  of  mackerel  (Scomber)  after  Allis.     For  letters  see  fig.  68. 

The  chondrostei,  the  most  shark-like  of  the  Ganoids,  have  no  cranial  cartilage 
bones.  They  are  also  primitive  in  the  great  development  of  the  rostral  cartilage 
(enormous  in  Polyodon),  which  gives  the  mouth  its  ventral  position,  and  in  the 


Fig.  77. — Chondrocranium  of  Polypterus,  after  Budgett.  a,  afferent  artery  to  external 
gills;  b^*,  branchials;  e,  efferent  artery  from  external  gills;  lb,  labial  cartilage;  2,  5,  7, 
nerve  exits;  other  letters  as  in  fig.  69. 

extension  of  the  cranial  cavity  into  the  ethmoid  region.  They  have  a  few  bones  in 
the  visceral  skeleton,  while  there  are  numerous  membrane  bones  in  the  roof  of 
the  skull,  a  few  of  them  readily  homologized  with  those  of  other  vertebrates. 


SKELETON. 


79 


In  other  ganoids  (holosteans  and  crossopterygians)  the  skull  is  much  like  that 
of  the  teleosts,  differing  in  the  extension  forward  of  the  cranial  cavity.     There  are 


Fig.  78. — Median  section  of  skull  of  mackerel  (Scomber)  after  Allis.     For  letters  see  fig.  68. 

one  {Amia)  or  two  (Polypterus)  gular  bones  developed  between  the  rami  of  the 
lower  jaw,  and  in  Polypterus  parietals,  frontals  and  nasals  fuse  with  age,  and  there 
are  numerous  small  bones  in  the  cranial  roof,  developed  along  the  lateral  line 
canals.  Amia  has  several  splenials  in 
the  lower  jaw. 

Teleosts  (fig.  76-80)  have  a  consider- 
able range  of  skull  structure.  In  the  lower 
groups  like  siluroids  and  cyprinids,  the 
chondrocranium  is  largely  persistent  and 
the  cranial  cavity  extends  into  the  eth- 
moid region  as  in  the  higher  ganoids. 
In  other  teleosts  the  trabeculae  are  ap- 
proximate between  the  orbits  (tropibasic) 
and  develop  a  thin  interorbital  septum 
which  limits  the  anterior  ends  of  the 
cranial  cavity.  The  cartilage  bones  are 
more  numerous.  All  four  occipitalia  are 
present,  the  occipital  condyle  being  formed 
by  basi-  and  exoccipitals.  Basi-,  all-,  and 
orbitophenoids  occur,  and  besides  ecteth- 
moids  a  pair  of  mesethmoid  ossifications. 
In  the  otic  capsule  there  are  usually 
pterotic  and  sphenotic  ossifications. 

The  cranial  roof  is  largely  formed  by 
the  frontals  and  parietals,  the  latter  fre- 
quently separated  by  a  strong  process  of 
the  supraoccipital.  Several  of  the  car- 
tilage bones  are  visible  from  above.  The 
roof  of  the  mouth  is  formed  by  the  large 
parasphenoid  and  the  vomers.  Premaxil- 
laries  (rarely  lacking)  and  maxillaries  yig.  79.— Dorsal  view  of  skull  of  mack- 
form  the  upper  jaw,  both  usually  bearing  erel.  Scomber,  after  Allis;  letters  as  in  fig.  68 


8o 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


teeth,  but  occasionally,  by  overdevelopment  of  the  premaxillary,  the  maxillary  is 
excluded  from  the  margin  of  the  jaw. 

Instead  of  the  single  pterygoid  of  higher  vertebrates  there  are  three  bones,  an 
entopterygoid  adjoining  the  palatine,  a  mesopterygoid  (ectopterygoid)  which  ex- 
tends back  to  the  quadrate,  and  a  metapterygoid  above  the  quadrate.  When  the 
hyomandibular  cartilage  ossifies  it  forms  a  hyomandibular  bone  from  its  upper  por- 
tion and  a  sjmiplectic  (an  element  not  known  outside  the  teleostomes),  which  sup- 
ports the  quadrate.  A  small  bone,  the  interhyal,  intervenes  between  the  hyomanr 
dibular  and  the  rest  of  the  hyoid.  The  hyoid  copula  consists  of  several  elements, 
the  anterior,  which  supports  the  tongue  being  called  the  entoglossal,  the  posterior, 
which  connects  with  the  branchial  arches,  the  urohyal.  The  fifth  gill  arch  consists 
of  a  single  element  on  either  side,  the  hypopharyngeal  bone,  which  usually  bears 


Fig. 


80. — ^Pterygoids,  suspensorium  and  operculum  of  mackerel  {Scomber)  after  Allis. 
For  letters  see  fig.  68. 


teeth,  the  two  sides  being  fused  in  the  plectognaths,  forming  a  pharyngeal  jaw. 
The  upper  elements  of  the  other  arches  are  frequently  expanded,  bear  teeth,  and 
are  called  epipharjmgeal  bones. 

Dipnoi. — In  the  three  existing  genera  the  skull  is  comparatively  uniform,  but 
the  fossils,  beginning  in  the  Devonian,  have  a  wide  range  of  structure.  In  the 
former  the  cavity  of  the  chondrocranium  extends  to  the  ethmoid  region  and  the  nasal 
capsules  have  a  second  opening,  corresponding  to  the  inner  nares  (choanae)  inside 
the  oral  cavity.  The  pterygoid  is  fused  with  the  cranium  (autostylic)  and  there  are 
one  (Protopterus)  or  two  (Ceratodus)  labial  cartilages  connected  with  the  nasal 
capsules.  In  Ceraiodus  there  are  no  cranial  cartilage  bones,  but  in  the  other 
genera  a  plate  composed  of  fused  ex-  and  supraoccipitals  occurs. 

The  membrane  bones  are  few,  but  their  homologies  are  not  always  certain.- 
The  roof  is  largely  formed  by  an  unpaired  bone  in  the  position  of  frontals  and 
parietals,  in  front  of  which  is  a  median  bone  (supraethmoid  or  fused  nasals)  above 
the  nasal  capsules.  In  Ceratodus  a  bone  of  uncertain  homology  occurs  on  either 
side  of  the  fronto-parietal,  but  it  is  lacking  in  the  others,  unless  it  be  represented  in 
Protopterus  by  a  pair  of  bones  which  abut  against  the  supraethmoid  and  overlap 


SKELETON. 


8l 


the  fronto-parietals.  The  otic  capsule  and  quadrate  are  covered  by  a  squamosal, 
and  the  roof  of  the  mouth  is  formed  by  a  large  parasphenoid,  in  front  of  which  are 
a  pair  of  palatines.  In  advance  of  these  last  are  a  pair  of  large  teeth  resting  directly 
on  cartilage,  their  bases  representing  the  greatly  reduced  vomers.  The  lower  jaw 
has  three  bones  on  either  side,  a  small  dentary,  a  larger  angulare,  and  an  enormous 
splenial,  which  alone  bears  teeth. 

In  Ceratodus  there  is  a  hyomandibular  fused  to  the  cranium  behind  the  exit  of 
the  seventh  nerve,  but  elsewhere  there  is  only  the  hyoid.  The  operculum  has  one 
or  two  elements  (operculare  and  interoperculum)  the  free  edges  of  which  bear 


Fig.  8i. — Skull  oi Lepidosiren,  after  Bridge,  an,  angulare;  ap,  antorbital  process;  ch 
ceratohyal;  cr,  cranial  rib;  de,  dermal  ethmoid;  dee,  dermal  ectethmoid;  eo,  exoccipital;/^ 
frontoparietal;  hr,  hyoidean  ribs;  mk,  Meckel's  cartilage;  na,  first  neural  arch;  nc,  nasal 
capsule;  nsp,  neural  spine;  pq,  pterygoquadrate;  sc,  sagittal  crest  of  frontoparietal;  sp 
splenial;  sq,  squamosal;  i-io,  nerve  exits. 


cartilaginous  rays,  and  the  gill  arches  are  five  in  Ceratodus,  six  in  the  other  genera. 
A  peculiar  feature  of  Protopterus  and  Lepidosiren  is  the  so-called  head  rib,  a  slender 
cartilage  bone  articulated  with  the  chondrocranium  below  the  occipital  plate,  and 
extending  backward  and  downward  across  the  shoulder  girdle. 

In  those  extinct  Dipnoi  which  are  united  with  the  recent  genera  to  form  the  order 
Sirenoidea,  the  skull  is  much  as  in  the  existing  forms,  except  for  the  more  numer- 
ous bones.  In  the  Arthrodira  (formerly  called  placoderms)  the  cranium  is  hinged 
to  a  large  plate  which  covers  the  anterior  part  of  the  trunk,  and  the  skull  is  roofed 
with  a  few  large  plates,  some  of  which  may  be  homologized  with  those  of  the  siren- 
oids,  the  others  not  being  readily  compared  with  the  bones  of  other  vertebrates. 
The  suggestion  has  been  made  that  the  problematic  fossil  Palceospondylns  resembles, 
in  its  skull,  the  larvae  of  the  dipnoans,  the  adults  of  which  were  common  in  the  same 
seas. 

6 


82 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


AMPHIBIA. — Several  points  distinguish  the  amphibian  from  other  skulls.  The 
chondrocranium  is  platybasic  (p.  6i) ;  except  for  a  small  synotic  tectum  frequently 
present,  it  is  not  roofed  by  cartilage;  the  otic  capsule  has  a  fenestra  vestibuli  occu- 
pied by  the  stapes,  a  development  connected  with  the  power  of  hearing  (p.  73) ; 
there  are  two  occipital  condyles;  and  the  quadrate  is  immovably  united  to  the 
cranium  by  two  processes,  an  otic  process,  joining  the  otic  capsule,  and  an  *  ascend- 
ing process'  which  reaches  the  upper  margin  of  the  trabecula,  and  which,  in  many 
reptiles,  often  ossifies  as  the  epipterygoid  bone. 


Fig.  82. — Chondrocranium  of  Amphiuma,  lateral  and  dorsal  views,  aop,  antorbital 
process;  ap,  ascending  process  of  quadrate  (epipterygoid) ;  ct,  cornua  trabeculae;  c?e, 
foramen,  for  ductus  endolymphaticus;  ep,  ethmoid  plate;/o,  fenestra  vestibxili ;  m,  Meckel's 
cartilage;  n,  notochord;  oc,  olfactory  capsule;  ov,  occipital  vertebrae;  p,  parachordal;  5, 
quadrate;  s,  stapes;  t,  trabecula;  2-8,  nerve  exists. 


The  cartilage  cranial  bones  are  few.  Usually  only  exoccipitals  are  developed 
in  the  hinder  region,  while  the  rule  is  a  single  petrosal  (prootic),  but  occasionally 
epi-,  opisth-,  and  pterotic  occur.  There  is  but  a  single  pterygoid,  while  basi-,  pre-, 
and  alisphenoids  are  not  ossified.  The  membrane  bones  in  existing  amphibians 
have  separated  from  the  integument  and  have  sunk  to  a  deeper  position  than  in 
fishes,  but  in  the  stegocephals  the  presence  of  grooves  for  the  lateral  line  system 
would  indicate  a  close  connexion  between  skin  and  bones.  In  the  latter  group  the 
membrane  bones  are  numerous,  but  in  existing  species  they  are  noticeably  reduced. 
Except  in  stegocephals  and  the  caecilians  there  are  large  vacuities  in  both  floor  and 
roof  of  the  skull.  The  lower  jaw  also  has  a  reduced  number  of  bones,  there  being 
at  most  five  including  the  articulare  and  the  mento-Meckelian. 

The  most  primitive  conditions  occur  in  the  stegocephals,  where,  as  the  name 


SKELETON. 


83 


indicates,  the  dorsal  surface  is  covered,  leaving  only  gaps  for  the  eyes  and  nostrils. 
In  general  the  account  of  the  skull  given  on  page  67  ff  will  apply  to  these  forms,  and 
so  far  as  the  dorsal  surface  is  concerned  little  more  needs  to  be  said,  aside  from  the 
fact  that  the  supratemporal  is  sometimes  transversely  divided,  that  an  interparietal 
foramen  occurs  (indicating  the  existence  of  a  parietal  eye),  that  the  bones  called 
supraoccipital  may  be  interparietal,  and  that  the  sclerotics  are  common.  The 
floor  of  the  cranium  is  formed  by  a  large  parasphenoid,  bordered  in  front  by  a  pair 
of  (usually  toothed)  palatines,  in  front  of  which  are  the  vomers.  Of  the  carti- 
laginous parts  almost  nothing  is  known;  a  few,  clearly  larval  forms  have  well 
developed  branchial  arches  preserved. 


Fig.  83 . — Skull  of  a  stegocephalan  {papitosaurus)  after  Zittell.     Letters  as  in  fig.  68. 

Of  the  Gymnophiones  (caecilians)  the  cartilage  skull  is  known  only  in  Ichthy- 
ophis;  its  peculiarities  are  the  reduced  parachprdals,  an  ethmoidal  nasal  septum, 
a  stapes,  perforated  as  in  mammals,  and  alisphenoid  and  trabecular  cartilages  more 
distinct  than  in  most  amphibia.  Most  noticeable  of  the  cartilage  bones  is  the  eth- 
moid, while  otics  and  exoccipitals  are  fused  as  are  quadrate  and  pterygoid.  The 
membrane  bones  form  a  complete  roof  to  the  skull,  recalling  the  stegocephals,  but 
the  number  of  bones  is  smaller,  squamosal,  supratemporal,  jugal  and  quadrato- 
jugal  being  absent,  while  a  large  prefrontal  and  a  larger  postfrontal  (usually  called 
squamosal)  occur.  In  the  roof  of  the  mouth  maxillary  and  palatine  are  fused,  the 
vomers  distinct,  while  the  united  parasphenoid  and  basioccipital  form  a  large  os 
basale.  In  the  lower  jaw  there  are  only  dentary  and  angulare,  the  latter  being 
produced  behind  the  articulare  in  a  remarkable  way. 

In  the  cartilage  skull  of  the  Urodeles  (fig.  82)  the  pterygoid  does  not  usually 
reach  the  anterior  part  of  the  skull  but  projects  as  a  process  from  the  quadrate, 
which  bears,  besides  the  two  processes  already  mentioned  (p.  82),  a  palatobasal 


84 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


process  joining  the  otic  capsule  in  front  of  the  otic  process.  Cartilage  bones  are 
few;  supra-  and  basioccipital,  alisphenoid  and  ethmoids  are  lacking;  the  otics  fuse 
to  a  single  petrosal;  an  orbitosphenoid  occurs  and  quadrate  and  pterygoid  are 
continuous. 

The  roof  of  the  adult  skull  is  chiefly  formed  of  parietals,  frontals  and  nasals, 
the  latter  being  frequently  separated  by  processes  of  the  premaxillaries.     Each 


Fig.  84. — Skull  of  Amblystoma  punciatum,  after  Wiedersheim.     Letters  as  in  fig.  68. 

frontal  has  a  ventral  process  which  limits  the  cranial  cavity  in  front;  there  is  usually 
a  prefrontal  and  a  septoiiiaxillary  may  be  developed  on  the  postero-lateral  part  of 
the  nasal  capsule.  A  supratemporal  is  always  lacking,  the  squamosal  extending 
up  to  the  parietal.  The  upper  jaw  is  composed  of  premaxillaries  and  (except  some 
perennibranchs,  fig.  85)  maxillaries;  a  jugal  is  always  absent  and  the  quadratojugal, 


Fig.  85. — Skull  of  Proteus,  after  Wiedersheim.     For  letters  see  fig.  68. 

formed  in  the  larva,  fuses  with  the  squamosal.  In  the  roof  of  the  mouth  are  the 
large  parasphenoid,  frequently  with  teeth,  and  a  pair  of  vomero-palatines,  the 
choanae  lying  behind  the  vomerine  portion,  which  is  farther  back  than  in 
the  dipnoi. 

In  the  lower  jaw  Meckel's  cartilage  persists,  its  hinder  end  forming  the  articulare, 
while  in  front  it  is  surrounded  by  the  dentary  and  splenial,  each  bearing  teeth.  In 
the  larvae  the  branchial  skeleton  is  nearly  typical,  there  being  a  hyoid  and  four  gill 
arches.    In  the  adult,  with  the  loss  of  aquatic  respiration,  the  posterior  arches  are 


SKELETON. 


85 


reduced  or  even  disappear,  those  remaining  being  connected  by  a  one  or  two-jointed 
copula. 

The  chondrocranium  of  the  larval  Anura  (Rana,  fig.  86)  differs  considerably 
from  that  of  other  amphibia  as  well  as  from  the  adult  conditions.  Like  all  amphib- 
ians it  is  platybasic.  The  pterygoquadrate  has,  besides  the  normal  otic  and 
epipterygoid  processes,  a  cranio-quadrate  process  connected  with  the  nasal  region, 
in  front  of  which  is  the  articulation  of  the  lower  jaw.  In  front  of  the  comua  are  a 
pair  of  suprarostral  cartilages  and  a  similar  pair  of  infrarostrals  lie  in  front  of  the 


Fig.  86. — Chondrocranium  of  tadpole  of  Rana  before  the  metamorphosis;  after  Gaupp. 
c.,ant,  anterior  canal;  els,  superior  labial  cartilage;  dr,  comu  trabeculae;  car,  foramen  for 
carotid;  exi.  c,  external  canal; /<?,  ethmoid  fenestra;  m,  Meckel's  cartilage;  pc,  posterior 
canal;  po,  otic  process  of  quadrate;  pr.  as.'  ascending  process  of  quadrate;  q,  quad- 
rate; /w,  tectmn  medialis;  Urn,  taenia  tecti  marginalis;  tsyn,  tectum  synoticum;  I-V,  nerves 
and  nerve  exits. 


Meckelian,  from  which  they  are  apparently  derived.  These  four  rostrals  form  a 
ring  around  the  suctorial  mouth  and  recall  the  labial  cartilages  of  the  elasmobranchs 
and  the  annular  cartilage  of  the  cyclostome  mouth. 

At  the  time  of  metamorphosis  the  changes  are  great,  and  as  the  result  is  more 
like  the  chondrocranium  of  other  amphibia,  the  larval  condition  must  be  regarded 
as  adaptive  rather  than  ancestral.  The  suprarostrals  disappear  and  the  jaw 
shifts  the  hinge  back  to  the  normal  position,  this  being  accompanied  by  the  elon- 
gation of  Meckel's  cartilage,  an  absorption  of  the  ascending  process  and  a  folding 
of  the  pterygoquadrate  bar.  At  the  same  time  a  pterygoid  grows  out  in  front  to 
join  an  antorbital  process  from  the  cranium.     A  stapes  develops  and  connects 


86 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


with  the  columella,  which  meets  the  tympanic  membrane.  This  membrane  is 
stretched  on  a  cartilaginous  tympanic  annulus,  derived  from  the  pterygoquadrate. 
(Annulus  and  columella  are  lacking  in  those  genera,  Bombinaior,  etc.,  which  have  no 
tympanum).     There  is  no  connexion  between  stapes  and  quadrate. 

The  chondrocranium  largely  persists,  the  only  constant  cartilage  bones  being 
the  exoccipitals  and  prootics.     A  supraoccipital  rarely  occurs  and  basioccipital  and 


t-  trried 


Fig.  87. — Chondrocranium  of  a  frog  after  metamorphosis,  from  Gaupp.  fov^  fenestra 
ovalis;  m,  Meckel's  cartilage;  mtg,  metapterygoid;  wc,  nasal  capsule;  ptgq,  pterygoquadrate; 
tnas,  tectum  nasalis;  tsyn,  tectum  synoticum;  ttmed,  tsenia  tecti  medialis. 


basisphenoid  are  unknown.  In  the  ethmoid  region,  except  in  the  aglossa,  there  is 
a  peculiar  bone,  the  sphenethmoid,  which  arises  as  two  bones  on  either  side. 
These  fuse,  forming  a  ring  (*os  en  ceinture')  around  the  olfactory  nefves  and  the 
anterior  end  of  the  brain. 

The  frontals  and  parietals  of  a  side  are  fused  and  often  the  fronto-parietals  are 
continuous  across  the  middle  line.     They  may  extend  to  the  nasals  or  there  may 


Fig.  88.- 


-Dorsal  and  ventral  views  of  skull  of  toad,  Bufo  americanus. 

fig.  68. 


For  letters  see 


be  a  gap  between,  leaving  the  sphenethmoid  visible  from  above.  A  large  squamosal 
extends  above  the  quadrate,  from  the  otic  region  to  the  angle  of  the  jaw.  The  upper 
jaw  consists  of  premaxillary  and  maxillary,  and,  except  in  the  aglossa,  of  quadrato- 
jugal.  The  pterygoid  cartilage  persists,  but  is  overlaid  by  a  membrane  bone,  also 
called  the  pterygoid.  Slender  palatines,  transverse  to  the  axis  of  the  skull,  are 
lacking  only  in  the  aglossa,  while  small  vomers  are  almost  always  present.     The 


SKELETON. 


87 


floor  of  the  cranium  is  completed  by  a  X-shaped  parasphenoid,  which  extends  to 
the  premaxillaries  in  the  aglossa,  elsewhere  only  to  the  sphenethmoid. 

In  the  lower  jaw  there  is  a  mento-Meckelian  in  front,  followed  by  dentary  and 
angulare,  Meckel's  cartilage  persisting  through  life.  The  larval  branchial  and 
hyoid  arches  are  typical,  there  being  four  gill  arches.  With  the  loss  of  gills  the 
posterior  arches  disappear,  and  the  broad  hyoid  plate  of  the  adult  has  four  processes 
which  are  new  formations. 

REPTILES. — The  skull  of  existing  reptiles  is  very  different  from  that  of  amphib- 
ians, but  that  of  many  theriomorphs  is  strikingly  like  that  of  the  stegocephalans. 
The  principal  differences  alluded  to  in  the  first  sentence  have  arisen  by  reduction 
and  disappearance  of  bones  appearing  in  the  more  primitive  types,  but  aside  from 
these  there  is  little  except  the  parasphenoid  to  separate  the  two  groups. 


Fig.  89. — Chondrocranium  of  Sphenodon,  stage  'R,'  after  Howes  and  Swinnerton. 
ep,  epipterygoid;  e$,  ethmosphenoidal plate;  «c,  extranasal  cartilage;  exp,  extranasal process; 
h,  hyoid;  mk,  Meckel's  cartilage;  nc,  nasal  capsule;  oc,  otic  capsule;  pt,  pterygoid;  g,  quad- 
rate; sb^  subnasal  process;  1-5,  exits  of  nerves. 

The  chondrocranium  is  known  in  but  a  few  forms  and  these  agree  with  other 
amniotes  in  being  tropibasic,  except  in  snakes  and  amphisbaenans  (see  fig.  62). 
In  the  adults  cartilage  largely  disappears,  except  in  the  ethmoid  region,  more 
persisting  in  Sphenodon  (fig.  89)  and  the  lizards  than  elsewhere.  All  four  occipitalia 
are  ossified,  but  some  may  not  participate  in  framing  the  foramen  magnum,  the 
basioccipital  being  excluded  in  many  chelonians,  the  supraoccipital  in  snakes, 
crocodiles  and  theriomorphs.  There  is  but  a  single  occipital  condyle  (except  in  a 
few  theriomorphs),  which  is  borne  on  the  basioccipital  as  in  the  crocodiles,  or  on 
this  and  the  exoccipitals  as  in  chelonians  and  squamata.  Basi-  and  presphenoids 
are  present,  orbito-  and  alisphenoids  are  but  slightly  ossified  and  the  ethmoid  region 
is  largely  cartilaginous.  Pro-,  epi-  and  opisthotics  are  present,  the  epiotic  fusing 
with  the  supraoccipital,  while  the  opisthotic  in  all  recent  forms  except  the  turtles 
unites  with  the  exoccipital  in  the  adult. 

In  all  except  the  squamata,  in  which  it  is  movable  (streptostylic),  the  quad- 


88         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

rate  is  firmly  united  to  the  squamosal  and  sometimes  to  other  bones  (monimostylic). 
The  pterygoids  extend  forward  to  the  palatines.  In  the  squamata  and  the  ichthyo- 
saurs  pterygoids  and  palatines  are  widely  separated  in  the  middle  line,  but  else- 
where they  are  closely  approximate,  the  pterygoids  even  meeting  the  basisphenoid. 
In  all  except  chelonians,  some  dinosaurs  and  the  typhlophida  an  ectopterygoid 
(os  transversum)  extends  from  pterygoid  to  maxilla,  while  in  plesiosaurs  and 
most  lizards  (kionocraniate)  ossification  of  the  ascending  process  of  the  quadrate 
forms  an  epipterygoid  bone  between  the  pterygoid  and  the  parietal. 

Membrane  bones  are  more  numerous  than  in  the  amphibians.  In  many 
theriomorphs  there  is  a  supratemporal  fossa  between  parietal  and  supratemporal 
bones  and  the  same  is  true  of  plesiosaurs,  ichthyosaurs  and  chelonians.  In  the 
rhynchocephals,  dinosaurs,  pterodactyls  and  crocodiles  there  is  in  addition,  an 
infratemporal  fossa,  bounded  laterally  by  an  arcade  in  which  squamosal,  quad- 
ratojugal  and  zygomatic  participate  in  varying  degrees.  In  the  lizards  the  two 
unite  in  a  single  temporal  fossa  by  the  disappearance  of  the  upper  arcade,  and 
lastly,  in  the  snakes  the  lower  arcade  is  lost  and  the  fossa  becomes  a  gap  in  the 
side  of  the  skull. 

Parietals  and  frontals  are  usually  paired,  a  parietal  foramen  being  common; 
pre-  and  postfrontals  usually  occur,  sometimes  excluding  the  frontal  from  the  orbit. 
Lacrimals  are  common  and  the  margins  of  the  upper  jaw  are  formed  in  front  by 
premaxilla  and  maxillary,  the  latter  connected  with  the  squamosal,  sometimes 
by  jugal  and  quadratojugal,  or  the  jugal  may  drop  out,  or  lastly  the  jaw  may  end 
with  the  maxillary.  Several  membrane  bones  may  aid  in  the  formation  of  the 
roof  of  the  mouth.  There  is  a  small  parasphenoid  in  ichthyosaurs,  plesiosaurs, 
many  squamata,  some  rhynchocephals,  and  rarely  in  turtles.  It  is  usually  asso- 
ciated with  the  basisphenoid  and  in  ophidia  it  forms  the  base  of  the  interorbital 
septum.  The  vomers  are  paired  except  in  the  chelonia,  and  only  in  Sphenodon 
of  recent  species  do  they  bear  teeth,  and  here  but  one  on  each  bone.  The  maxil- 
laries  usually  have  broad  palatal  processes  extending  toward  the  middle  line,  causing 
the  choanae  to  open  farther  back,  and  in  some,  these,  together  with  the  palatines 
and  pterygoids,  form  a  false  palate,  ventral  to  the  nasal  passages,  so  that,  as  in  the 
crocodiles,  the  choanae  are  carried  far  back  in  the  mouth.  In  some  dinosaurs 
there  is  a  rostral  bone  in  front  of  the  premaxillae. 

The  two  halves  of  the  lower  jaw  are  united  by  ligament  in  most  rhynchocephals, 
snakes  and  pythonomorphs;  by  suture  in  crocodiles,  rhynchocephals  and  lizards; 
while  they  are  fused  in  turtles  and  pterosaurs.  All  of  the  bones  mentioned  on 
page  71  may  occur  in  the  lower  jaw,  usually  with  distinct  sutures,  while  in  croco- 
diles, theriomorphs  and  some  dinosaurs  there  are  gaps  or  vacuities  in  its  walls. 
In  many  dinosaurs  there  is  a  predentary  bone  at  the  tip  of  the  jaw.  Except  in 
the  chelonia  and  a  few  isolated  forms,  both  jaws  bear  teeth,  which  may  be  restricted 
to  maxillaries  and  premaxillaries,  or  may  also  occur  on  palatines,  vomers  and 
pterygoids.  In  their  fixation  three  types  are  found:  acrodont,  when  fused  to  the 
margin  of  the  bone;  pleurodont,  when  fastened  to  the  side  of  the  bone;  and  the- 
codont, when  implanted  in  sockets. 

The  hyoid  apparatus  is  much  modified,  but  is  adequately  known  only  in  recent 


SKELETON. 


89 


species.  The  branchial  arches  are  usually  better  developed  than  the  hyoid  proper, 
which  is  cartilaginous  in  most  snakes  and  is  lacking  in  the  crocodiles.  In  the 
chelonia  (fig.  93)  two  branchial  arches  are  usually  present. 

The  Theriomorphs  (fig.  90)  have  a  short,  broad  skull  with  parietal  foramen; 
and  that  of  the  cotylosaurs  was  much  like  that  of  the  stegocephals.  In  the  more 
differentiated  groups  the  skull  recalls  that  of  mammals,  especially  in  the  partici- 
pation of  the  squamosal  in  the  hinge  of  the  jaw.  Lacrimals  are  occasionally 
absent,  sclerotics  sometimes  present.  The  palatal  region  is  known  in  a  few  forms. 
The  pterygoids  may  meet  only  in  front,  lea\T[ng  a  vacuity  between  it  and  the 
basisphenoid,  or  they  may  meet  that  bone.  The  choanae  are  in  front  of  the  pal- 
atines but  (theriodonts)  may  be  displaced  backward  by  palatine  processes  of  the 
maxillaries. 

All  four  occipitalia  are  developed;  the  occipital  condyle  is  tripartite,  being  formed 
by  basi-  and  exoccipitals,  but  in  Cynognathus  the  recession  of  the  basioccipital 
results  in  a  dicondyHc  condition.     The  greatest  variations  occur  in  the  temporal 


Fig.  90. — Skull  of  Procolophan,  after  Woodward.     For  letters  see  fig.  68. 


region.  In  the  lower  cotylosaurs  the  cranial  roof  is  without  fossae  (Broom  doubts 
the  infratemporal  fossa  of  Procolophon).  In  other  theromorphs  quadratojugal 
and  supra  temporal  are  lacking,  the  squamosal  meeting  the  parietal.  Placodus 
has  only  the  supratemporal  fossa,  but  in  the  majority  the  upper  arcade  has  dis- 
appeared, leaving  a  large  temporal  vacuity,  much  as  in  mammals. 

Little  is  known  of  the  lower  jaw.  The  bones  are  sometimes  discrete,  sometimes 
extensively  fused.  The  teeth  are  thecodont,  and  in  the  theriodonts  are  differentiated 
into  incisors,  canines  and  molars,  but  in  the  anomodonts  teeth  are  absent,  or  at  most 
there  are  a  pair  of  large  incisors  in  the  upper  jaw. 

In  the  Plesiosaurs  and  their  allies  the  skull  is  about  a  twelfth  of  the  total  length. 
There  is  a  parietal  foramen  between  the  parietals,  which  have  a  process  for  artic- 
ulation with  the  squamosal,  the  supratemporal  being  absent.  The  large  pre- 
frontals intervene  between  the  frontals  and  the  orbits;  lacrimals  and  usually 
nasals  are  absent.  The  large  temporal  fossa  in  bounded  externally  by  the  jugal 
which  extends  back  to  the  quadrate.  The  choanae  are  in  front  of  the  palatines;  an 
OS  transversum  is  present  and  there  is  frequently  a  parasphenoid  in  the  inter- 
pterygoid  vacuity.     All  have  a  subtemporal  vacuity  and  there  is  another  in  the 


90 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


Fig.  91. — Skull  of  Plesiosaurus  macrocephalus,  after  Andrews,  ang,  angulare;  art, 
articulare;  ch,  choana;/r,  frontal;  orb,  orbit;  pa,  parietal;  pal,  palatine;  pas,  parasphenoid; 
po+pof,  postorbital  and  postfrontal;  sf,  supratemporal  fossa;  /,  transversvim.  Other 
letters  as  in  fig.  68. 


mx 


Fig.  92. — Dorsal  and  ventral  views  of  the  skull  of  turtle,  Trionyx  ss,  exoccipital; 
m,  maxillary;  p,  (behind  naris),  preorbital;  pno,  postorbital;  s,  supraoccipital;  other 
letters  as  in  fig.  68. 


SKELETON. 


91 


plesiosaurs  in  the  angle  between  palatine  and  transversum.     The  usual  bones  are 
frequently  distinct  in  the  lower  jaw. 

In  the  Chelonians  the  cranial  cavity  extends  forward  between  the  eyes  and  the 
mesethmoidal  cartilage  largely  persists  in  the  adult.  Although  the  bones  are 
comparatively  few,  the  skull  is  primitive  and  can  only  be  derived  from  that  of  the 
cotylosaurs.  The  bones  are  firmly  united,  but  the  sutures  are  evident.  The 
basioccipital  is  usually  excluded  from  the  foramen  magnum,  and  it  and  the  ex- 
occipitals  participate  in  the  tripartite  occipital  condyle.  The  supraoccipital  is 
often  prolonged  into  an  occipital  spine  and  is  fused  with  the  epiotics.  The  basi- 
sphenoid  is  present,  but  pre-,  ali-  and  orbitosphenoids  are  not  ossified,  a  descending 
plate  of  the  parietal  taking  the  place  of  the  alisphenoid.     The  pterygoids  meet  the 


Fig.  93. — Hyoid  apparatus  of  Tryonyx.     b^,  b^,  first  and  second  branchial  arches;  bhy 
basihyal   (copula);  h,  reduced  hyoid;   cartilage  dotted. 

basisphenoid  and  may  extend  to  the  basioccipital.  No  ectopterygoid  is  present. 
The  monimostylic  quadrate  is  large  and  expanded  laterally  to  support  the  tympanic 
membrane,  and  notched  or  perforate  behind  for  the  columella. 

In  the  most  primitive  chelonians  a  complete  false  roof  is  formed  by  the  expanded 
postfrontals,  parietals  and  squamosals.  In  most  of  the  species  the  recession  of  the 
parietals  and  squamosals  causes  a  large  gap,  bounded  in  front  by  postfrontal  and 
jugal  and  exposing  the  otic  bones.  Laterally  this  gap  is  limited  by  an  arcade  of 
squamosal  and  quadratojugal,  but  the  latter  may  be  reduced  or  {Cistudo)  absent. 
In  front  of  the  frontals  are  a  pair  of  bones,  which  bound  the  single  naris  behind. 
These  occupy  the  position  of  lacrimals,  nasals  and  prefrontals,  and  are  called  by 
the  latter  name.  The  premaxillaries  are  usually  fused;  the  maxillae  have  broad 
palatal  processes  and  trenchant  margins.  They,  together  with  the  zygomatics, 
form  the  lower  border  of  the  orbit. 

The  vomer  is  a  single  vertical  plate  separating  the  two  choanae.  The  palatines, 
which  bound  the  choanae  behind  are  broad  and  are  firmly  united  to  pterygoids 
and  basisphenoid.     A  parasphenoid  is  known  only  in  Dermochelys.     In  the  lower 


92 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


jaw  the  bones  are  often  fused,  the  two  halves  being  united.  Again  the  bones  may 
be  distinct,  the  splenial  being  the  least  constant  element.  The  hyoid  apparatus 
consists  of  a  cartilaginous  copula  and  two  pairs  of  cornua  which  do  not  reach  the 
cranium. 

IcHTHYOSAURS  havc  a  short  temporal  region  but  elongate  nasals  and  pre-max- 
illaries  form  a  long  rostrum.     There  is  a  large  supratemporal  fossa  and  enor- 


FiG.  94. — Dorsal  (A),  posterior  (B),  ventral  (C),  and  lateral  (D)  views  of  the  skull 
of  Ichthyosaurus  longifrons,  after  Woodward,  nar,  naris;  pas,  parasphenoid ;  pmx,  pre 
maxilla;  ptf,  postfrontal;  pto,  postorbital.     Other  letters  as  in  fig.  68. 


mous  orbits,  bounded  above  by  pre-  and  postfrontals,  below  by  an  elongate  jugal, 
and  containing  a  sclerotic  ring.  The  nares  are  just  in  front  of  the  orbits  and  the 
parietal  foramen  is  at  the  junction  of  frontals  and  parietals.  All  four  occipitalia 
bound  the  foramen  magnum;  the  basisphenoid  is  short,  the  presphenoid  long;  and 
the  pterygoids  are  separated  in  front  by  the  vomers,  leaving  large  pterygoid  vacui- 


FiG.  95. 


-Side  and  posterior  views  of  skull  of  young  Sphenodon,  after  Howes  and  Swinner- 
ton.     Compare  with  fig.  69.     Cartilage  dotted;  letters  as  in  fig.  68. 


ties.  The  choanse  are  far  forward.  Teeth  (sometimes  absent)  occur  in  grooves. 
The  lower  jaw  has  five  or  six  distinct  bones,  and  a  rib-like  hyoid  has  been  found  in 
some  species. 

The  only  living  Rhynchocephalian  is  Sphenodon  (Hatteria)  of  New  Zea- 
land. It  is  lizard-like,  but  its  skull  (figs.  68,  95)  differs  in  the  three  temporal 
fossae,  the  infratemporal  arcade  being  osseous  as  in  no  lizard.  Then  the  quadrate 
is  anchylosed  to  pterygoid,  squamosal  and  quadratojugal.     Premaxillae,  maxillse  and 


SKELETON. 


93 


palatines  bear  teeth;  an  epipterygoid  is  present  and  the  lower  margin  of  the  orbit 
is  formed  by  the  maxillary.  In  the  extinct  genera  the  jugal  may  bound  the  orbit 
below  (Palaeohatteria),  and  the  vomer  may  bear  teeth. 

Dinosaurs  have  both  supra-  and  infratemporal  fossae  and  frequently  a  pre- 
orbital  vacuity  as  well.  The  rostral  and  predentary  bones  have  been  mentioned 
(p.  88).  The  palatal  region  recalls  that  of  Sphenodon,  except  that  the  teeth,  in 
grooves  or  sockets,  never  occur  on  the  palatines.  There  are  such  variations  in  the 
skulls  that  few  general  statements  can  be  made. 

Statements  that  will  apply  to  all  Squamata  are  few.  Except  in  chamaeleons 
the  quadrate  is  movable,  a  quadratojugal  is  lacking,  the  boundary  of  the  infra- 
temporal fossa  being  completed  by  ligament.     The  external  nares  are  separate,  there 


Fig.  96. — Skull  of  Gerrhonotus  imhricattts,  after  Siebenrock.     For  letters  see  fig.  68. 

are  large  vacuities  in  the  floor  of  the  skull  and  the  choanae  are  forward.  An 
ectopterygoid  occurs  except  in  the  typhlopids  and  all  four  occipitalia  bound  the 
foramen  magnum. 

The  chondrocranium  of  the  Lizards  (fig.  62),  while  much  like  the  general  type  of 
tropibasic,  is  very  light  and  is  fenestrated  to  an  extent  not  seen  in  the  ichthyopsids. 
Among  the  peculiarities  of  the  adult  skull  are  the  fusion  of  exoccipital  and  opisthotic 
to  form  a  'parotic  process'  which,  together  with  the  squamosal,  supports  the  quad- 
rate. There  is  a  looseness  of  connexion  of  the  front  of  the  skull  with  the  occipito- 
sphenoidal  portion,  these  parts  moving  on  each  other.  The  hyoid  apparatus 
bears  two  cornua  which  either  end  freely  in  the  neck  or  may  reach  the  parotic 
process. 

In  the  Pythonomorphs  the  striking  features  are  the  large  supratemporal  fossae, 
the  quadrate  recalling  that  of  chelonians;  and  the  joint  in  the  lower  jaw,  between 
dentary  and  angular  regions. 


94 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  Ophidia  (snakes)  lack  parotic  process,  parietal  foramen,  temporal  arcades 
and  epipterygoid,  and  have  the  squamosal  excluded  from  the  cranial  wall.  The 
attachment  of  the  visceral  skeleton  to  the  cranium  is  loose,  the  pterygoid  being 
connected  to  the  other  parts  by  a  long  bar,  consisting  of  squamosal  and  quadrate 
behind  and  by  transversum  and  palatine  in  front,  features  related  to  the  great 
distensibility  of  the  jaws.  In  the  poisonous  serpents  the  poison  fangs  are  either 
permanently  erect,  or  they  fold  back  when  the  mouth  is  closed.  In  the  latter  the 
fangs  are  supported  on  the  maxillaries,  which  are  moved  by  a  rod  formed  of  quad- 
rate, pterygoid  and  ectopterygoid.  In  the  lower  jaw  distensibility  is  provided  for 
by  the  elastic  ligament  connecting  the  two  halves  in  front.  Some  species  have 
remnants  of  the  hyoid  apparatus,  but  occasionally  all  are  lost  in  the  adult. 


as 


Fig.  97. — Skull  of  snake,  Tropidonotus,  after  W.  K.  Parker. 

When  the  whole  series  of  Crocodilia,  recent  and  extinct,  is  considered  the 
range  of  variation  in  the  skull  is  considerable.  In  all,  supra-  and  infratemporal 
fossae  are  present,  the  quadrate  is  immovable,  there  is  more  or  less  of  a  secondary 
palate,  no  parietal  foramen,  and  the  thecodont  teeth  are  confined  to  the  margins 
of  the  jaws.  In  the  complete  series  the  gradual  change  of  position  of  the  choanae 
can  be  traced  from  the  oldest  in  which  they  are  beside  the  vomers;  then  in  the  meso- 
suchia  the  palatines  meet  in  the  middle  line,  carrying  the  choanae  back  as  a  single 
opening  behind  these  bones;  while  in  the  recent  species  the  pterygoids  have  also 
met,  so  that  the  choanae  are  between  them  and  the  basisphenoid. 

Among  the  recent  species  the  basioccipital  is  excluded  from  the  foramen  mag- 
num, pre-  and  orbitosphenoids  are  imperfectly  ossified,  the  nasals  are  long  and  the 


SKELETON. 


95 


premaxillaries  short  so  that  the  nares  are  far  in  front;  parietals  and  usually  the 
frontals  are  fused  in  the  middle  line.  There  are  vacuities  in  both  walls  of  the 
lower  jaw,  which  is  also  pneumatic. 

Although  there  is  no  relation  between  the  two,  the  skull  of  the  Pterosaurs  is 
very  bird-like  in  its  length  and  in  having  its  axis  at  right  angles  to  that  of  the  body, 
while  the  elongate  premaxillae  form  a  bird-like  beak.  The  sutures  between  the 
bones  are  largely  obliterated  in  the  adult  and  the  brain  cavity  recalls  that  of  birds. 
The  resemblances  are  heightened  in  some  by  the  lack  of  teeth,  in  others  they  are 
in  sockets.  Both  supra-  and  infratemporal  fossae  are  present,  as  well  as  a  large 
preorbital  vacuity,  sometimes  united  with  the  naris.  Squamosal  and  quadrate 
are  inclined  forward  so  that  the  hinge  of  the  jaw  is  often  beneath  the  orbit.  There 
is  no  parietal  foramen  and  all  of  the  bones  of  the  jaw  are  fused,  including  those  of 
the  two  halves. 


Fig.  98. — Skull  of  Caiman  laiirostris,  based  on  a  figure  by  Reynolds;  the  irregularitesi  of 
the   surface   omitted.     Letters   as   in   fig.    68. 


AVES. — The  skull  of  birds  is  similar  in  many  respects  to  that  of  lizards.  The 
chondrocranium  arises  as  two  distinct  parts,  pre-  and  perichordal,  which,  on  account 
of  the  great  head  flexure,  are  at  an  angle  of  100°  to  each  other,  later  increased  to 
160°,  which  persists  through  life.  There  are  three  (or  four?)  occipital  vertebrae  be- 
hind the  ear,  the  last  being  the  most  prominent,  and  there  is  a  small  synotic  tectum. 
From  the  first  the  otic  capsules  are  continuous  with  the  basal  plate  and  the  fenestra 
vestibuli  is  formed  later  by  resorption  of  the  cartilage.  The  trabeculae  are  at  first 
distinct  from  each  other  as  well  as  from  the  perichordal  part;  later  they  fuse  in 
front  of  the  hypophysis  to  give  rise  to  the  base  of  the  interorbital  septum.  In 
Tinnunculus  the  ethmoid  plate  arises  early  as  an  intertrabecular  mass,  from  which, 
later,  the  dorsal  part  of  the  interorbital  septum  arises  as  a  backward  growth  of 
cartilage.  Large  alisphenoid  cartilages  are  connected  with  the  otic  capsules. 
The  nasal  capsules  are  complicated  and  later  give  rise  to  several  centres  of  ossi- 
fication. The  quadrate  is  free  from  the  rest  (streptostylic)  and  its  pterygoid  process, 
the  homologue  of  the  pterygoid  cartilage  in  other  groups,  is  greatly  reduced.  The 
other  visceral  arches  are  much  as  in  the  adult  (infra). 


96 


COMPARATIVE      MORPHOLOGY    OF    VERTEBRATES. 


The  bones  are  lighter  than  those  of  reptiles  and  are  often  pneumatic,  that  is, 
are  penetrated  with  canals  connected  with  the  respiratory  system.  The  brain 
cavity  is  larger  than  in  reptiles;  sutures  between  the  bones  largely  disappear  in  the 
adult,  and  the  single  occipital  condyle  (mostly  basioccipital)  is  on  the  floor  of  the 
skull  so  that  the  axis  of  the  skull  is  at  right  angles  to  that  of  the  body.  There  is 
only  a  single  temporal  fossa,  bounded  laterally  by  an  arcade  of  jugal  and  quad- 
ratojugal,  connecting  quadrate  and  maxillary.  There  is  a  preorbital  vacuity; 
and  the  nares  may  have  the  posterior  margin  rounded  (holorhinal)  or  slit-like 
(schizorhinal).     The  premaxillaries  are  fused  and  sclerotic  bones  are  common. 

A  peculiarity  of  the  ventral  surface  is  the  union  of  the  anterior  part  of  the 


Fig.  99. — Earlier  and  later  stages  of  skull  of  bird  (Tinnunculus)  after  Suschkin.  a/, 
alisphenoid  cartilage;  at,  foramen  for  internal  ophthalmic  artery;  b,  basal  plate;  bpt, 
basipterygoid ;  ec,  external  semicircular  canal;  hm,  'hyomandibular;'  iorb,  interorbital 
plate;  it,  intertrabecula;  mc,  middle  concha  of  nose;  ov,  occipital  vertebrae;  pc,  posterior 
semicircular  canal;  sorb,  supraorbital;  str,  supratrabecula;  tr,  trabecula. 

parasphenoid  to  the  basisphenoid  to  form  a  *rostrum  sphenoidale*  which  projects 
forward  in  the  middle  line.  The  rest  of  the  parasphenoid  forms  a  *basitemporal 
plate'  below  the  basisphenoid  and  basioccipital.  Dorsal  to  the  rostrum  is  a 
small  presphenoid  (sometimes  lacking  in  the  adult)  to  which  the  orbitosphenoids  are 
attached  as  alae,  while  the  alisphenoids  become  similar  wings  to  the  basisphenoid. 
Ectethmoids  are  connected  with  the  mesethmoid;  they  are  sometimes  large,  appear- 
ing ('prefrontals')  on  the  top  of  the  skull.  Epi-  and  ectopterygoids  are  lacking. 
The  pterygoids,  here  membrane  bones,  extend  from  the  quadrates  to  the  palatines, 
and  the  two  either  slide  along  the  rostrum  or  the  vomers  intervene.  This,  together 
with  the  hinging  of  the  front  part  of  the  skull  upon  the  rest,  forms  a  mechanism  by 
which  the  upper  jaw  is  raised  when  the  mouth  is  opened,  the  temporal  arcade  aiding 
in  the  motion.  The  vomers  may  be  paired;  usually  they  form  a  thin  vertical  plate 
between  the  anterior  ends  of  the  pterygoids;  occasionally  they  disappear.  The 
choanae  are  between  the  palatines  and  vomers.     Some  birds  have  an  *os  lincin- 


SKELETON. 


97 


atum,'  a  small  bone  connecting  the  lacrimal  with  the  palatine  or  jugal  bar.     All 

of  the  bones  enumerated  on  page  71  may  appear  in  the  development  of  the  lower 

jaw. 

Teeth  occur  only  in  a  few  fossil  birds,  where  they  are  implanted  in  sockets; 

several  species  are  known  to  have  a  dental  ridge  in  the  embryo  (see  Development 
,     '  of    Teeth).     The   hyoid  apparatus   (fig.    loi) 

consists  of  a  pair  of  cornua  (first  branchials) 
sometimes  extremely  long,  connected  by  the 
hyoid  copula  (os  entoglossum),  behind  which 
is  a  second  copula  (urohyal)  while  in  front  of 
the  entoglossum  is  a  ^paraglossaP  element 
with  a  pair  of  small  cornua. 

The  palatal  structures  have  considerable 
importance  in  classification.  All  living  birds 
can  be  arranged  in  two  groups.  In  the  *dro- 
maeognathous'  group  the  palatines  and  ptery- 


FiG.   100. — ^\'entral  view  of  skull 
of  a  duck;  letters  as  in  fig.  68. 


Fig.  ioi. — Hyoid  of  hen,  after  Parker. 
e,  entoglossal;  />,  paraglossal;  u,  urohyal; 
///,  posterior  cornua. 


golds  do  not  articulate  with  the  rostrum,  the  vomers  usually  intervening.  In 
the  *euomithes'  the  articulation  occurs.  The  latter  are  subdivided  into  the 
desmognathous  forms  where  the  vomer  is  small  or  wanting,  and  the  maxillo- 
palatines  meet  in  the  middle  line;  the  schizognathous  in  which  the  maxillo- 
palatines  do  not  meet  the  vomer  or  each  other;  the  aegithognathous,  like  the 
last  except  that  the  vomer  is  broad  and  truncate;  and  the  saurognathous 
with^deKcate,  rod-like  vomers  and  maxillopalatines  scarcely  extending  inwards 
from  the  maxillaries. 
7 


98 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  chondrocranium  of  the  MAMMALS  has  several  peculiarities.  There  are 
four  occipital  vertebrae,  the  last  only  with  a  complete  vertebral  character,  all  event- 
ually fusing  with  the  synotic  tectum.  The  dorsal  part  of  the  otic  capsule  chondrifies 
first,  owing  to  the  late  development  of  the  cochlear  part  of  the  ear  in  the  lower  half; 
and  the  capsules  themselves  have  their  axes  inclined,  so  that  the  exit  of  the  seventh 
nerve  is  on  the  anterior  rather  than  on  the  lateral  face.  The  trabeculae  soon  join 
the  basal  plate  and  from  their  sellar  part  an  alary  process  is  given  off  on  either  side 


Fig.  I02. — Chondrocranium  of  a  pig,  after  Mead,  as,  alisphenoid;  cl,  posterior  clinoid 
process;  cr,  fenestra  cribrosa;  end,  foramen  for  endolymph  duct;/w,  foramen  magnum;  h, 
fossa  h5rpophyseos;  Isr,  lateral  superior  recess;  os,  orbitosphenoid;  pi,  parietal  lamina;  sn, 
septum  nasi;  <»,  tectum  nasi;  2-12,  exils  of  nerves. 


which  extends  upward  to  join  an  alisphenoid  (ala  temporalis)  which  chondrifies 
separately,  but  soon  joins  the  otic  capsule  above,  leaving  between  them  the  foramen 
ovale  for  the  third  branch  of  the  fifth  nerve,  the  other  branches  passing  forward 
over  the  ala  and  then  between  it  and  the  orbitosphenoid  (ala  orbitalis)  through 
the  sphenoidal  fissure  (foramen  lacerum  anterior).  The  ala  orbitalis  joins  the 
trabecula  by  two  processes,  bar  and  processes  sometimes  forming  a  reduced  inter- 
orbital  septum.     Later  a  marginal  band  (taenia  marginalis)  extends  back  from 


SKELETON. 


99 


the  orbitosphenoid  to  a  cartilage  plate  developed  on  the  otic  capsule.  The  ethmoid 
parts  are  complicated,  consisting  of  the  two  nasal  capsules,  the  septum  between 
them,  and,  on  the  inside,  coiled  turbinal  cartilages  to  support  the  olfactory  membrane. 

Some  of  the  visceral  arches  have  been  mentioned  in  speaking  of  the  ear  bones 
(p.  74).  The  pterygoid  cartilage  is  apparently  lacking,  and  there  is  nothing  that 
can  be  interpreted  as  a  quadrate  except  the  incus.  Meckel's  cartilage  extends  for- 
ward from  the  incus  to  the  tip  of  the  jaw.  In  the  procartilage  stage  the  hyoid  is 
continuous  with  the  stapes;  later  it  joins  the  otic  capsule  behind  the  fenestra  ves- 
tibuli,  while  ventrally  it  joins  its  fellow  and  is  connected  with  the  first  branch- 
ial arch  by  a  median  cartilage,  probably  the  copula. 

In  the  adult  the  so-called  facial  bones  are  more  closely  related  to  the  cranium 
than  in  the  lower  groups,  and  distinct  bones  are  fewer  than  in  lower  vertebrates, 
the  reduction  being  due  in  part  to  actual  loss,  in  part  to  the  fusion  of  elements 


Fig.  103. 


-Diagram  of  the  bones  of  the  mammalian  skull,  altered  from  Flower, 
bones   dotted,    membrane   bones   lined;   2-12,    nerve   exits. 


Cartilage 


which  elsewhere  remain  distinct.  The  obliteration  of  sutures  has  gone  farther  in 
the  monotremes  and  some  of  the  carnivores  and  apes  than  elsewhere.  Connected 
with  the  loss  of  bones  is  the  absence  of  the  supratemporal  arcade,  but  the  infra- 
temporal bar  consisting  of  processes  from  the  squamosal  and  zygomatic  (malar) 
is  always  present,  bounding  the  single  temporal  fossa.  This  may  be  separated 
from  the  orbit  by  a  bar  formed  by  zygomatic  and  frontal,  or  the  bar  may  be  in- 
complete or  absent  so  that  orbit  and  fossa  are  one. 

Usually  the  bones  fuse  in  such  a  way  that  the  complexes  named  on  page  66  are 
readily  recognized.  The  occipitalia  are  usually  united  into  a  single  occipital  bone, 
though  the  sutures  between  them  may  persist  for  some  time.  The  basioccipital 
forms  the  so-called  basilar  process,  while  the  exoccipitals  bear  the  two  occipital 
condyles  for  articulation  with  the  atlas.  The  exoccipitals  may  also  bear  strong, 
ventrally  directed,  paramastoid  processes  (paroccipital).  The  membranous 
interparietal  is  sometimes  distinct,  sometimes  fused  to  the  supraoccipital,  though  it 
may  unite  with  the  parietals. 


lOO        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

The  sphenoidalia  form  the  sphenoid  bone  of  human  anatomy.  Basi-  and  pre- 
sphenoid  form  a  'body'  from  which  two  pairs  of  'wings'  arise,  the  alisphenoids 
being  the  greater,  the  orbitosphenoids  the  lesser  wings.  A  pair  of  pterygoid  pro- 
cesses are  given  off  from  the  ventral  side  of  the  body  and  a  part  of  these  in  some  cases 
persist  as  distinct  pterygoid  bones,  but  apparently  are  not  homologous  with  the 
elements  of  the  same  name  in  the  lower  vertebrates  since  they  are  membrane  bones. 
The  equivalents  of  the  pterygoids  of  the  non-mammals  occur  in  the  monotremes. 
A  second  pair  of  membrane  bones,  the  intertemporals,  also  belong  to  the  sphenoid 
complex,  fusing  at  an  early  date  with  the  dorsal  margin  of  the  alisphenoids. 

The  ethmoid  complex  consists  of  a  mesethmoid  which  ossifies  in  the  septum 
between  the  nasal  organs,  and  an  ectethmoid  in  the  outer  wall  of  each  nasal  capsule. 
Mes-  and  ectethmoids  are  distinct  for  a  time,  the  olfactory  nerve  passing  between 
them.  Later  bony  strands  passing  between  the  nerve  fibres  unite  them,  producing 
perforated  cribiform  plate,  characteristic  of  the  mammals.  The  part  of  the 
mesethmoid  projecting  above  the  cribiform  plates  is  the  cristi  galli,  below  them  is 


Fig.  104. — Median   section    of   skull   of   young  Erinaceus,    after  I'arkcr.     For  letters  see 

fig.  68. 

the  perpendicular  plate.  Two  other  centres  in  the  lateral  wall  of  each  capsule 
give  rise  to  coiled  bones  (inferior  and  sphenoidal  turbinal)  on  which  the  olfactory 
membrane  is  spread,  while  two  other  turbinals  (superior  and  middle)  arise  from 
the  ectethmoid.  A  few  mammals  have  in  addition,  a  prenasal  bone,  developed 
in  the  septum  in  front  of  the  mesethmoid. 

The  temporal  complex  consists  of  squamosal,  otic  bones  and  tympanic.  On 
the  ventral  side  of  the  squamosal  is  the  glenoid  fossa  for  the  articulation  of  the 
lower  jaw;  in  front  the  bone  gives  ofiF  a  zygomatic  process  for  articulation  with  a 
similar  process  of  the  zygomatic  (malar)  bone,  the  two  forming  the  arcade  bounding 
the  temporal  fossa.  The  tympanic  (apparently  the  angulare  of  the  lower  vertebrates) 
curves  below  the  auditory  meatus,  joining  the  squamosal  on  either  side.  In  many 
forms  it  expands  to  form  a  large  capsule,  the  auditory  bulla.  The  otic  bones 
(it  is  said  that  there  are  six  centres  of  ossification  in  the  otic  capsule)  unite  early 
to  form  a  single  petrosal  bone,  which,  in  turn  (cetaceans  excepted)  fuses  with  squa- 
mosal to  form  the  temporal  bone.  Later,  the  posterior  part  of  the  otic  region  expands 
to  form  the  mastoid  process,  while  the  upper  part  of  the  hyoid,  fused  to  the  cap- 
sule, forms  a  styloid  process. 

On  account  of  the  great  size  of  the  brain  some  parts  of  the  skull  are  changed  in 


SKELETON.  lOI 

position.  Thus  the  petrosal,  instead  of  forming  part  of  the  side  wall,  is  carried  to 
the  floor  of  the  brain  cavity  and  the  squamosal  forms  part  of  the  lateral  wall. 
The  roof  of  the  brain  cavity  is  largely  formed  by  parietals  and  frontals.  (In  some 
whales,  denticetes,  the  supraoccipital  and  interparietal  extend  to  the  frontal,  pre- 
venting the  parietals  from  meeting.)  The  frontals  may  be  distinct  or  they  may 
fuse.  In  many  ungulates  they  bear  horns  or  antlers.  In  catde,  antelopes,  sheep 
and  goats  (cavicornia)  a  strong  bony  process  or  horn  core  is  developed  on  each 
frontal,  and  this  is  covered  by  a  cornified  epidermis  and  persists  through  life.  The 
antlers  of  the  deer  differ  from  horns.  Each  year  there  is  an  outgrowth  of  bony 
material,  covered  by  a  richly  vascular  skin,  from  each  frontal  bone.  This  grows 
with  remarkable  rapidity,  and  when  its  full  extent  is  reached,  the  skin  ('velvet')  is 
lost,  leaving  the  core  alone.  After  about  a  year  resorption  takes  place  at  the  base 
so  that  the  antler  is  soon  lost,  to  be  replaced  by  a  similar  but  larger  one  in  a  few 
weeks. 

The  nasals  lie  above  and  behind  the  nares.  The  margin  of  the  upper  jaw  is 
formed  by  premaxillaries  followed  by  the  maxillaries  which  ossify  from  several 
centres,  difficult  to  homologize  with  distinct  bones  in  the  lower  vertebrates.  The 
inferior  turbinals  fuse  to  the  inner  surfaces  of  the  maxillaries.  Premaxillaries  and 
maxillaries  may  fuse  or  they  may  remain  distinct.  They  have  broad  palatine 
processes  on  the  oral  surface,  these  meeting  in  the  middle  line  and  forming  the 
anterior  part  of  the  hard  palate,  with  frequently  one  or  two  incisive  foramina 
for  the  passage  of  the  nasopalatine  nerve  between  them.  The  choanae  are  usually 
behind  the  palatine  bones  which  form  the  rest  of  the  hard  palate,  but  in  some  eden- 
tates and  whales  the  pterygoids  form  part  of  the  partition  between  the  narial  pas- 
sages and  the  mouth  cavity. 

The  ingrowth  of  the  hard  palate  has  forced  the  vomer  from  the  roof  of  the  mouth 
to  a  position  just  ventral  to  the  anterior  part  of  the  cartilage  of  the  nasal  septum. 
In  the  monotremes  there  is  a  'dumb-bell 
bone*  in  front  of  the  vomer  (p.  69).  A 
lacrimal  bone  always  occurs  at  the  inner 
side  of  the  orbit  and  the  zygomatic  forms 
the  external  wall  of  that  cavity. 

The  lower  jaw  articulates  directly  with 
the  squamosal  without  the  intervention  of 
a  quadrate  (see  ear  bones,  p.  74).  Its 
halves  may  unite  in  front  by  ligament  or  ^^^   io5.-Hyoid  of  rhi^ceros  (Ate- 

by  complete  anchylosis.  It  is  usually  lodus).  ac,  anterior  comu;  b,  body;  c, 
described  as  consisting  of  a  pair  of  den-  ceratohyal;er,epihyal:^<;,  posterior  comu 
taries,  but  there  are  several  centres  of  ossi-     ^    >^    ^   -'• 

fication  and  a  splenial  and  possibly  a  coronoid  may  be  recognized.  The  angulare 
is  apparently  the  tympanic,  while  the  articulare  of  lower  vertebrates  is  the  malleus. 
A  remarkable  feature  in  development  is  an  enormous  cartilage  at  the  posterior 
angle  of  the  jaw,  the  dorsal  side  of  which  forms  the  condyle  for  articulation  with 
the  glenoid  fossa. 

The  hyoid  apparatus  varies.  As  described  above,  the  hyoid  is  connected  above 
with  the  otic  region,  below  with  the  first  branchial.     The  part  connected  with  the 


I02 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


otic  capsule  forms  the  styloid  process  (p.  loo),  while  the  rest  may  ossify  as  epi-, 
cerato-,  and  hypohyals,  or  a  part  may  change  to  a  stylohyal  ligament,  connecting 
the  ventral  parts  with  the  skull.  The  hyoid  of  the  adult  consists  of  the  copula 
forming  the  body,  a  part  of  the  hyoid  the  anterior  cornua,  while  the  first  branchial 
arch  (of  which  at  most  but  one  or  two  HhyrohyaP  elements  are  formed)  give  rise 
to  the  posterior  cornua.  These  are  connected  by  ligament  with  the  greatly  modified 
posterior  branchial  arches,  described  in  connection  with  the  larynx  (see  respiratory 
organs). 

Appendicular  Skeleton. 

The  appendages  fall  in  two  categories,  the  median  or  azygos 
(median  fins)  found  only  in  aquatic  vertebrates  and  the  paired  appen- 
dages, which  (cyclostomes  excepted)  are  found  in  every  class,  although 
here  and  there  individual  species  or  genera  may  lack  them.  Both 
kinds  have  an  internal  skeleton.  Opinions  differ  as  to  the  origin  of 
these  appendages.     The  two  most  prominent  views  are  given  below. 


Fig.  io6. — Diagram  of  the  origin  of  median  and  paired  appendages  from  lateral  fin  folds. 

According  to  one  view  the  two  types  have  no  relation  to  each  other.  The 
paired  appendages  are  derived  from  gill  septa,  all  traces  of  which  are  otherwise 
lost  from  these  somites.  The  girdles  which  support  the  appendages  are  modified 
gill  arches,  while  the  skeleton  of  the  appendage  itself  is  derived  from  the  radialia 
which  support  the  gills,  one  radial  forming  an  axis,  the  adjacent  radials  being 
arranged  on  either  side  of  this,  and  carried  outward  from  the  arch  by  the  growth 
of  the  septum  to  form  the  body  of  the  appendage  (fig  122).  A  somewhat  similar 
view  is  that  the  appendage  itself  is  a  modification  of  an  external  gill,  such  as  is 
found  in  larval  amphibians. 

Another  view  supposes  an  ancestor  with  two  pairs  of  longitudinal  folds  running 
the  length  of  the  body  behind  the  head,  each  fold  supported  by  a  series  of  skeletal 
rods  (fig.  106).     With  farther  development  the  upper  folds  on  either  side  migrated 


SKELETON.  IO3 

dorsally  until  the  two  met  and  fused  in  the  middle  line  of  the  back,  thus  producing 
a  continuous  dorsal  fin.  The  ventral  folds  migrated  downward  in  the  same  way, 
eventually  meeting  behind  the  vent,  but  that  opening  prevented  their  meeting 
farther  forward.  From  the  fused  part  behind  the  vent  the  anal  and  the  lower  part 
of  the  caudal  fins  were  formed,  while  the  paired  appendages  are  differentiations  of 
the  preanal  parts  of  the  ventral  longitudinal  folds. 

It  may  be  said  that  in  development  there  is  no  such  double  origin  of  the  dorsal 
fin.  In  several  sharks  the  paired  fins  arise  from  continuous  folds,  while  in  the 
Japanese  gold  fish  the  anal  fins  are  frequently  paired  and  the  caudal  has  a  double 
condition  below,  such  as  would  result  from  the  failure  of  folds  to  unite  in  this  region. 
In  criticism  of  the  gill-arch  theory  it  may  be  said  that  the  supports  of  the  paired 
appendages  arise  outside  of  the  body  musculature,  while  the  visceral  arches  (p.  65) 
are  internal. 

The  Median  Appendages. 

The  median  or  azygos  appendages  always  have  the  form  of  fins, 
and  may  be  dorsal,  terminal  (caudal)  or  ventral  (anal)  in  position. 
Primitively,  and  in  many  species  through  life,  they  are  continuous,  but 
usually  gaps  occur  during  development  so  that  the  fins  of  the  adult  are 
separated  by  intervals  from  each  other.  They  occur  in  practically  all 
fishes,  in  larval  and  tailed  amphibians,  and  in  isolated  groups  like  the 
ichthyosaurs  and  whales.  In  amphibians  and  higher  groups  the 
median  fins  have  no  skeleton,  but  elsewhere  it  is  of  cartilage,  bone, 
or  a  horny  substance  (elastoidin) ,  the  latter  being  the  most  constant 
and  occurring  in  connection  with  either  of  the  others. 

The  simplest  skeleton  consists  of  a  metameric  series  of  cartilage  or 
osseous  bars,  each  usually  divided  into  a  deeper  basale  and  a  more 
distal  radiale,  the  former  frequently  articulating  or  alternating  with 
the  spinous  processes  of  the  vertebrae,  while  the  latter  support  the  fin 
proper.  The  elastoidin  elements  consist  of  a  number  of  slender  rods 
(actinotrichia) ,  outnumbering  the  somites,  and  arising  from  the 
corium,  immediately  below  the  epidermis.  Frequently  they  are 
united  into  bundles  (soft  fin  rays)  and  may  replace  the  radialia. 

Paired  Appendages. 

The  paired  appendages  are  not,  as  the  gill-arch  theory  would  demand,  derived 
from  a  single  somite,  but  a  varying  number  of  segments  participate  in  their  forma- 
tion. Apparently  the  simplest  fin  known  is  that  of  the  extinct  shark,  Cladoselache 
(fig.  107),  in  which  it  is  a  rounded  lobe  supported  by  a  number  of  rods,  like  the 
radialia  in  a  median  fin.     These  are  attached  proximally  to  a  few  larger  plates,  the 


I04 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


basalia,  the  basalia  of  the  two  sides  being  unconnected  with  each  other.  Greater 
growth  of  the  basalia  would  result  in  some  of  them  meeting  and  fusing  in  the  mid- 
dle line,  thus  forming  a  bar  across  the  ventral  side  of  the  body,  giving  additional 
support  to  the  fin.     Then  to  compensate  for  the  rigidity,  the  basals  become  jointed 

on  either  side,  leaving  the  medial  bar 
with  an  articular  surface  on  either  side 
for  the  reduced  basalia.  The  ventral 
muscles  of  the  fin  would  find  firm  at- 
tachment to  the  bar,  while  the  need  for 
a  similar  attachment  for  the  dorsal 
results  in  an  extension  of  the  bar  dor- 
sally  above  the  articulation  of  the 
limb,  thus  producing  the  typical  girdle. 
The  derivation  of  the  fin  of  any  fish 
from  that  of  Cladoselache  is  easily 
imagined,  but  no  satisfactory  compari- 
son of  the  fin  with  the  leg  has  yet  been 
made. 


In  the  appendicular  skeleton 
the  internal  supports  or  girdles 
and  the  skeleton  of  the  free  ap- 
pendage are  to  be  recognized. 
Each  girdle  is  an  inverted  arch 
crossing  the  ventral  side  of  the 
body  and  extending  up  on  either 
side  above  the  articulation  of 
the  limb.  The  girdles,  as  well 
as  the  skeleton  of  the  free  ap- 
pendage, are  aWays  laid  down 
in  cartilage,  and  in  the  latter, 
aside  from  the  actinotrichia,  no 
parts  of  other  than  cartilaginous 
orgin  occur.  In  the  girdles  mem 
brane   bones   may  be  added  as 

Fig.    107.— Ventral    surface    of  Cladoselache,    will  appear  below. 

after   Jaeckel.  j^^  -^^  typical  State  each  girdle 

consists  of  three  elements,  one  dorsal  and  two  ventral,  meeting  at  the 
point  of  attachment  of  the  free  appendage,  all  contributing  to  the 
socket  (glenoid  fossa,  acetabulum)  which  receives  the  basal  element  of 
the  skeleton  of  the  limb.  The  limbs  themselves  are  much  alike  in 
their  general  structure,  as  may  be  seen  from  the  adjacent  diagram. 


SKELETON. 

The  Shoulder  Girdle. 


lO: 


FISHES. — The  pectoral  or  shoulder  girdle  in  the  elasmobranchs  is 
more  or  less  U-shaped,  the  bottom  of  the  arch  crossing  the  ventral 
surface  between  the  skin  and  the  peritoneal  membrane,  this  ventral 
portion  being  known  as  the  coracoid  region,  which  is  limited  dorsally 


Fig.  io8. — Diagram  of  girdles  and  appendages  from  the  posterior  side;  upper  letters, 
fore  limb;  lower,  hind  limb,  a,  acetabulum;  c,  carpus;  co,  coracoid,  /,  femiu:;  fi,  fibula; 
g,  glenoid  fossa;  h,  humerus;  il,  ilium;  is,  ischium;  mc,  nU,  metacarpals,  metatarsals;  p, 
pubis;  pc,  procoracoid;  ph^-^^  phalanges;  r,  radius;  s,  scapula;  m,  ulna;  1-5  digits. 

by  the  point  of  attachment  (glenoid  fossa)  of  the  fin.  Dorsal  to  the 
fossa  is  the  scapular  region.  Not  infrequently  the  dorsal  part  of 
the  scapular  region  is  segmented  off  as  a  separate  suprascapula. 


Fig.  ioq. — Pectoral  girdle  and  cartilaginous  fin  skeleton  oiScyllium.  c,  coracoid  region ; 
gl;  glenoid  surface;  ms,  mesopterygium;  mt,  metapterygium;  p,  propterygium;  r,  radialia: 
5,  scapular  region. 

The  girdle  is  usually  free  from  the  axial  skeleton,  but  in  the  skates 
(raiae)  the  suprascapula  articulates  with  the  adjacent  vertebrae. 

In  the  simpler  teleostomes  (some  ganoids,  dipnoans)  the  cartilagin- 
ous girdle  is  reinforced  by  membrane  bones  derived  from  the  skin. 


I06        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

Of  these  there  are  at  least  two  on  either  side,  a  pair  of  clavicles  which 
overlie  the  coracoid  region  and  meet  in  the  middle  line,  and  lateral  to 
each  clavicle  and  extending  to  or  above  the  glenoid  fossa,  a  second 
bone,  the  cleithrum.  In  some  ganoids  {Polypterus,  fig.  no)  the 
cleithra  extend  toward  the  middle  line,  and  a  little  higher  in  the  scale, 
meet  and  take  the  strains.  This  assumption  of  stress  by  the  membrane 
bones  results,  in  the  higher  forms,  in  the  separation  of  the  two  halves 
of  the  cartilaginous  girdle. 

In   the  higher  ganoids  and  teleosts  the  cleithrum  has  increased 
greatly,  usurping  the  function  of  the  clavicles,  which  have  consequently 


Fig.  1 10. — ^Pectoral  girdles  of  (^1)  Acipenser  and  (B)  Polypterus,  after  Gegenbaur.     ct, 
cleithrum;  cv,  clavicula;  rfr,  dermal  rays;  g,  glenoid  surface. 

disappeared.  Dorsal  to  the  cleithra  other  membrane  bones  frequently 
occur.  There  may  be  one  or  two  supracleithra  (post-  or  supra- 
temporals,  fig.  79)  which  connect  the  girdle  with  the  skull,  and 
occasionally  others  as  postclavicle,  infraclavicle,  etc.  As  a  result 
of  the  great  development  of  the  cleithra  the  cartilaginous  girdle  has  been 
reduced,  but  it  usually  has  at  least  two  ossifications  on  either  side,  a 
scapula  dorsal  to  the  glenoid  fossa  and  a  coracoid  in  the  ventral  region, 
these  contributing  to  the  support  of  the  appendage. 

AMPHIBIA. — In  the  stegocephals  the  cartilage  has  not  been 
preserved  and  the  bones  are  variously  interpreted  (fig.  58) .  The  bone 
meeting  the  episternum  is  the  clavicle,  and  lateral  to  this  is  an  equally 
slender  bone,  usually  called  scapula,  but  by  some  the  cleithrum.     A 


SKELETON.  IO7 

large  round  element  is  called  the  coracoid.  In  the  recent  amphibians 
we  are  on  firmer  ground.  The  halves  of  the  girdle  develop  separately, 
and  the  cleithrum  is  lacking.  In  urodeles  the  coracoid  region  has  two 
processes  diverging  from  the  glenoid  fossa,  an  anteriorly  directed  pro- 
coracoid  and  a  coracoid  proper,  directed  toward  its  fellow  of  the 
opposite  side,  the  two  meeting  the  sternum  behind  and  overlapping  in 
front.  Ossification  sets  in  in  the  neighborhood  of  the  glenoid  fossa, 
the  resulting  bone  being  called  the  scapula,  although  it  invades  the 
coracoid  region,  the  cartilage  dorsal  to  it  being  the  suprascapula. 

In  the  toads  and  allied  anura  (arcifera)  the  halves  of  the  girdle 
overlap  as  in  the  urodeles,  but  the  procoracoids  extend  toward  the 
middle  line,  each  being  joined  to  its  coracoid  by  longitudinal  cartilage 
plate,  the  epicoracoid,  leaving  a  gap  between  them.  With  the  ap- 
pearance of  bone,  scapula  and  coracoid  ossify,  while  a  clavicle  of  mem- 


FlG.  III. — Arciferous  girdle  of  Ceratophrys  ornatus.     cl,  clavicle;  co,  coracoid;  e,  epicora- 
coid; h,  head  of  humerus;  s,  scapula;  55,  suprascapula;  cartilage  dotted. 

branous  origin  overlies  the  procoracoid  cartilage.  In  the  frogs  (firmi- 
sternia)  the  relations  are  much  the  same,  except  that  the  epicoracoids, 
instead  of  overlapping,  abut  against  each  other,  and  the  clavicles  nearly 
or  quite  replace  the  procoracoid,  while  sternum  and  omostemum  join 
the  girdle  in  front  and  behind.  Girdles  are  lacking  in  the  gymnophiones. 
REPTILES. — With  the  development  of  a  considerable  neck  in  the 
reptiles  the  pectoral  girdle  is  removed  farther  from  the  head;  it  shows 
considerable  differences  in  the  various  groups.  In  the  fossil  rhyn- 
chocephals  it  is  much  as  in  the  stegocephals,  except  that  the  scapula 
is  large.  In  the  turtles  it  occupies  a  peculiar  position,  being  inside 
the  carapace,  i.e.,  internal  to  the  ribs;  but  this  is  explained  by  the  de- 
velopment; the  girdle  arises  in  front  of  the  ribs  and  later  sinks  to  the 
definitive  position.  Scapula,  procoracoid  and  coracoid  are  well 
developed,  the  medial  ends  of  the  latter  two  being  connected  by  a  cartil- 
aginous epicoracoid.  Elsewhere  in  the  reptiles  the  procoracoid  tends 
to  reduction,  the  clavicle  taking  its  place,  though  it  is  retained  in  the 
lizards  in   a  reduced   condition  (fig.  112).     The  clavicle  in  turn  is 


io8 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


lost  in  chamaeleons  and  crocodiles,  and  if  present  in  the  chelonians,  it  is 
represented  by  the  epiplastron  (p.  41),  an  element  of  the  carapace. 
The  girdles  are  greatly  reduced  in  the  limbless  lizards  and  have  van- 
ished in  the  ophidians. 

In  the  BIRDS  (fig.  53)  the  scapula  is  a  sword-shaped  bar  overlying 
the  ribs,  while  the  coracoid  extends  from  its  junction  with  the  scapula 
at  the  glenoid  fossa  to  the  anterior  end  of  the  sternum.  The  clavicles 
of  the  two  sides  are  united  at  their  medial  or  ventral  ends  to  form  the 
well-known  furcula  (wishbone)  which  may  articulate  with  the  sternum 
between  the  two  coracoids,  or,  with  diminishing  powers  of  flight,  may 
end  freely  below. 


Fig.  112.  Fig.  113 

Fig.    1X2. — Sternum  and   pectoral  girdle  of   Amblyrhynchus,   after   Steindacher.     c, 

coracoid;  cl,  clavicle;  e,  epicoracoid;  es,  epistemum;  h,  humerus;  m,  mesocoracoid ;  ms, 

mesoscapula;  p,  procoracoid;  sc,  scapula;  s,  sternum. 

Fig.  113. — Shoulder  girdle  of  Ornithorhynchus .     cl,  clavicle;  co,  coracoid;  e,  epister- 

num;  g,   glenoid  fossa;  pc,   procoracoid;  s,   scapula;   st,   sternum. 


MAMMALS. — The  shoulder  girdle  of  the  monotremes  is  strikingly 
like  that  of  lizards,  the  coracoids  acting  as  a  brace  between  sternum  and 
glenoid  fossa,  while  the  resemblance  is  strengthened  by  the  presence  of 
the  episternum.  This  same  large  development  of  the  coracoids  occurs 
in  the  young  of  some  marsupials,  but  in  the  adults,  as  in  the  rest  of  the 
mammals,  the  coracoid  is  greatly  reduced,  persisting  only  as  a  small 
projection,  the  coracoid  process,  anchylosed  to  the  ventral  end  of  the 


SKELETON.  IO9 

scapula,  where  it  often  forms  a  part  of  the  glenoid  fossa.  The  scapula 
is  always  well  developed,  and  in  the  placental  mammals  bears  a  strong 
crest  (spina  scapulae)  on  its  external  surface,  terminating  ventrally  in 
an  acromion  process.  The  clavicle  varies  with  the  freedom  of  motion 
of  the  limb.  Thus  in  rodents,  insectivores,  bats,  some  marsupials  and 
the  higher  primates  it  forms  a  strong  brace  between  shoulder  and  ster- 
num. In  ungulates,  whales,  and  a  few  carnivores  it  has  entirely  dis- 
appeared, while  in  other  mammals  it  persists  as  a  rudiment  without 
functional  value.  In  development  two  small  elements  frequently 
intervene  between  the  clavicles  and  the  sternum  (fig.  55).  They 
are  preformed  in  cartilage  but  eventually  fuse  with  the  sternum. 
Their  homology  is  very  uncertain.  They  have  been  called  episternalia, 
suprasternalia,  etc. 

The  Pelvic  Girdle  {Pelvis). 

In  its  broader  features  the  pelvis  {cf.  fig.  108)  is  much  like  the 
shoulder  girdle,  and  in  its  full  development,  may  be  compared,  part  by 
part,  with  the  anterior  arch.  Thus  the  acetabulum  or  socket  where  the 
appendage  is  attached,  is  comparable  to  the  glenoid  fossa.  Dorsal 
to  this  is  the  ilium  in  the  position  of  the  scapula,  while  ventral  and 
medial  to  the  acetabulum  are,  on  either  side,  an  os  pubis  in  front,  an 
ischiimi  behind,  with  a  gap  (ischio-pubic  fenestra)  between  them,  just 
as  between  coracoid  and  procoracoid.  An  important  landmark  is 
the  point  of  passage  of  the  obturator  nenx  through  the  pelvis.  This 
may  have  its  own  (obturator)  foramen,  though  the  pubic  portion  or 
the  foramen  may  unite  with  the  fenestra,  the  condition  in  the  mammals 
where  the  common  opening  is  called  the  obturator  foramen. 

The  phylogenetic  history  of  the  pelvis  is  more  clearly  indicated  than 
is  that  of  the  pectoral  girdle,  for  in  many  fossils,  as  well  as  in  the 
sturgeon,  there  is  little  advance  over  Cladoselache  (p.  104).  The 
basalia  of  a  side  have  fused  to  a  single  basal,  often  perforated  for  the 
obturator  nerve,  and  bearing  the  radialia  on  its  distal  surface.  The 
basalia  of  the  two  sides  have  not  met,  but  there  is  frequently  between 
them  a  pair  of  small  cartilage  plates,  possibly  the  homologues  of  the 
epipubis  of  the  tetrapoda  (infra).  There  is  no  acetabular  joint.  In 
the  other  ganoids  and  in  teleosts  there  is  little  advance,  aside  from  ossifi- 
cation of  parts,  while  no  epipubic  elements  occur.  A  noticeable 
feature  in  many  acanthopterygians  is  the  forward  migration  of  the  pelvic 


no 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


fins  SO  that  they  come  to  lie  in  front  of  the  pectorals  (the  old  group 
of  'jugulares'). 

The  elasmobranchs  have  a  true  girdle,  but  without  separate  ele- 
ments as  it  does  not  pass  beyond  the  cartilage  stage.  It  consists  of  a 
continuous  ischio-pubic  bar,  extending  from  one  acetabulum  to  the 
other,  and  usually  prolonged  dorsally  above  the  acetabulum  by  an 
iliac  process. 

In  all  fishes  the  pelvic  girdle  is  free  from  the  vertebral  column,  but 
in  the  tetrapoda,  where  the  limbs  have  to  support  the  body  weight,  the 
girdle  becomes  connected  with  the  sacrum  by 
the  intervention  of  one  or  more  sacral  ribs  (p. 
56).  In  the  interpretation  of  some  of  the 
pelvic  elements  there  is  some  uncertainty. 

In  the  stegocephals  ischium  and  ilium  and 
usually  pubis  were  distinct  bones  with  appar- 
ently considerable  cartilage  between  them.     In 


Fig.  114.  Fig.  115. 

Fig.  114. — ^Pelvis  of  Discosaurus,  after  Credner.     il,  ilium;  is,  ischium;  p,  pubis. 

Fig.  115. — ^Ventral  view  of  pelvis  and  ypsiloid  cartilage  of  Cryptohranchus,  after  Wieder 
sheim.  a,  acetabulum;  il,  ilium;  is,  ischium;  0,  obturator  foramen;  p,  conjoined  pubes; 
y,  jrpsiloid  cartilage. 


the  urodeles  the  two  ischio-pubic  cartilages  are  usually  united  in  the 
median  line,  but  the  ossifications  vary  in  extent,  the  pubic  region  lagging 
behind  the  ischium  and  being  at  times  indistinguishably  fused  with  it. 
In  some  cases  there  is,  as  in  Necturus,  an  extension  of  the  median 
cartilage  forward  in  an  epipubic  process,  and  frequently  a  pectineal 
process  from  the  antero-lateral  of  each  pubis.  An  interesting  feature 
is  furnished  by  the  ypsiloid  cartilage  (fig.  115)  formed  independently 
of  the  pubis  and  extending  forward  in  the  linea  alba  through  two  or 
three  somites.  This  occurs  only  in  salamanders  with  functional  lungs, 
where  it  furnishes  attachment  for  muscles  connected  with  respiration. 
In  the  anura  all  three  pelvic  bones  are  present,  and  all  participate  in 
the  formation  of  the  acetabulum.     Correlated  with  the  leaping  habits 


SKELETON. 


Ill 


the  ilium  is  very  long  and  the  ischio-pubis  is  strongly  compressed, 
obturator  foramen  and  ischio-pubic  fenestra  being  absent. 

Omitting  the  extinct  rhynchocephals,  whose  pelvis  resembles  that  of 
the  stegocephals,  the  reptiles  have  the  pelvic  bones  more  solid  and  dis- 
tinct than  do  the  ichthyopsida;  the  ilium  is  strong,  with  its  dorsal  end 
frequently  expanded;  the  ischio-pubic  fenestra  is  large;  and  ischium 
and  pubis  are  united  to  their  fellows  directly,  or  by  the  intervention  of 
the  epipubic  cartilage,  or  its  modification,  the  ligamentum  medium 
pelvis.  As  a  rule  all  three  bones  meet  in  the  acetabulum  and  there 
are  large  prepubic  processes,  though  these  are  small  in  the  lizards  and 
are  lacking  in  crocodiles. 


Fig.  ii6.  Fig.  117. 

Fig.  116. — Pelvis  of  snapping  turtle  {Chelydr a)  {rom  below,  e,  epipubis;/,  femur; 
hy  hypoischium;  /,  ligamentum  medium  pelvis;  p,  pubis;  pp,  pectineal  process. 

Fig.  117. — ^Pelvis  of  Iguana  tuberculaia,  after  Blanchard.  a,  acetabulum;  e,  epipubic 
cartilage;/,  femur;  U,  ilium;  is,  ischium;  of,  obturator  foramen;  p,  pubis;  pp,  prepubis; 
J,*  s^,  first  and  second  sacral  vertebrae. 

Many  theriomorphs  have  the  pelvic  bones  fused  much  as  in  mam- 
mals. In  Sphenodon  and  turtles  the  epipubic  cartilage  bounds  the 
fenestra  on  the  median  side,  and  Sphenodon  and  the  plesiosaurs  have 
a  separate  obturator  foramen,  but  the  two  are  merged  in  the  chelonians. 
Most  lizards  have  slender  pubic  bones,  perforated  by  the  foramen,  and 
the  part  of  the  epipubis  between  the  fenestras  reduced  to  a  ligament, 
while  the  posterior  part  of  this,  behind  the  ischium,  may  ossify  as  a 
distinct  bone  (os  cloacaB  or  hypoischium).  In  the  footless  lizards  the 
pehds  is  reduced,  being  represented  in  the  amphisbaenans  by  rudiments 
of  ischium  and  pubis,  while  all  traces  of  the  pelvis  are  lost  in  snakes, 
except  the  boas  and  some  opoterodonts.  The  obturator  foramen  is 
very  large  in  the  crocodiles,  the  result  of  the  oblique  position  of  the 


112 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


pubes,  which  do  not  unite  with  each  other;  each  is  tipped  with  car- 
tilage (?  separate  epipubes).  All  three  bones  meet  at  the  acetabulm 
which  is  perforate  in  recent  species.  The  lower  end  of  the  ilium  sepa- 
rates as  a  distinct  bone  (pars  acetabularis). 

The  pelvis  of  the  dinosaurs  has  the  same  great  extension  of  the  ilium 
forward  and  back  as  is  seen  in  the  birds  and  a  corresponding  in- 
crease of  the  sacrum  (p.  53),  the  result  of  the  partially  upright  position. 


Fig.  118. 


-Pelvis  and  hind  limb  of  Cantptosaurus,  after  Marsh.    /,  femur;  fb,  fibula;  il, 
ilium;  is,  ischium;  p,  pubes;  pp,  postpubis;  t,  tibia;  I-IV,  digits. 


The  ischia  are  greatly  elongate  and  are  directed  backward,  being  fre- 
quently united  below.  The  pubic  bones  are  remarkable  in  being 
directed  forward  and  downward  and  in  ^having  strong  postpubic 
processes  which  are  parallel  to  the  ischium.  Frequently  the  ilium 
gives  off  an  iliac  spine  near  the  acetabulum. 

The  pterodactyls  had  the  same  elongate  ilium  as  the  dinosaurs,  the 
ischium  being  fused  to  it  so  as  to  exclude  the  pubis  from  the  acetab- 
ulum, the  latter^  being  usually  loosely  articulated  to  the  ischium  and 

^  This  pubis  is  sometimes  regarded  as  a  prepubis,  the  ischium  being  called  an  ischio- 
pubis. 


SKELETON. 


113 


meeting  its  fellow  in  the  median  line  below.  The  pehac  opening  was 
very  small.  The  pelvic  bones  of  the  ichthyosaurs  were  weak,  long  and 
slender,  and  apparently  were  imbedded  in  the  muscles. 

In  recent  birds  (figs.  50,  53)  the  pelvic  bones  are  fused.     The  ilium 
is  greatly  elongate  and  usually  fused  with  the  synsacrum  (p.  53) ;  ischium 


Fig. 


119. 


Fig.  120. 
A,  chick  of  6  days. 


Fig.  119. — Development  of  pelvis  of  chick,  after  Miss  Johnson. 
B,  older;  C,  20  days;  cartilage  dotted,  bone  white,     a,  acetabulum;  il,  ilium;  is,  ischium; 
in,  ischiadic  nerves;  on,  obturator  nerve;  p,  pubis;  pp,  pectineal  process. 

Fig.  120. — Pelvis  of  Galeopithecus,  after  Leche.     ah,  acetabular  bone;  i,  ischium;  i/, 
ilium;  p,  pubis;  cartilage  dotted. 


and  pubis  directed  backward.  The  pubes,  lying  in  the  position  of  the 
postpubes  of  the  dinosaurs,  never  meet  below  except  in  the  ostriches. 
In  the  embryo  (fig.  119)  they  are  at  first  directed  forward  and  only 
attain  the  final  position  later.  A  pec- 
tineal process  arises  from  the  aceta- 
bular region  and  extends  forward,  simu- 
lating the  dinosaur  pubis. 

In  the  mammals,  obturator  foramen 
and  ischio-pubic  fenestra  are  united, 
the  opening  being  bounded  on  the 
medial  side  by  processes  from  ischium 
and  pubis.  All  three  bones  may  meet  in 
the  acetabulum,  but  more  often  the  ex- 
tension of  ilium  and  ischium  excludes 
the  pubis  from  the  fossa.  A  peculiarity  is  the  common  occurrence  of  an 
additional  bone  in  the  formation  of  the  acetabulum  (acetabular  or  coty- 
loid bone) .  This  lies  between  ilium  and  pubic  bone  and  may  fuse  with 
any  of  the  elements.    In  marsupials  and  monotremes  the  interpubic  car- 


FiG.  121. — Left  side  of  pelvis  of 
duck-bill,  Ornithorhynchus,  a,  ace- 
tabulum; il,  ilium;  is  ischiimi;  w, 
marsupial  bone;  of,  obturator  fora- 
men; />,  OS  pubis;  sv,  sacral  vertebra. 


114        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

tilage  persists  for  some  time,  or  through  life,  but  elsewhere  it  disap- 
pears and  the  elements  unite  by  symphysis.  The  same  groups  of  non- 
placental  mammals  are  characterized  by  the  presence  of  marsupial 
bones  (fig.  121).  These  are  preformed  in  cartilage  and  extend  for- 
ward from  either  pubis  in  the  ventral  abdominal  wall.  Their  homol- 
ogy is  very  uncertain;  but  they  are  not  the  ypsiloid  of  the  urodeles. 


///////Ac, 


D 


Fig.  122. — Diagrams  illustrating  theories  of  origin  of  appendages.  A,'B,C,  origin  of 
biserial  appendage  (C)  from  gill  arch  {A)\D,  biserial  appendage  (archipterygium) ;  £,  F, 
evolution  of  elasmobranch  fin;  G,  dotted  lines  indicate  parts  involved  in  origin  of  leg  from 
fin;  iJ,  dotted  parts  show  another  view  of  origin  of  elements  of  leg. 

The  Free  Appendages. 

These  are  of  two  kinds,  the  paired  fins  (ichthyopterygia)  of  the 
fishes  and  the  legs  or  their  modifications  (chiropterygia)  found  in  all 
classes  of  tetrapoda.  The  former  is  merely  a  mechanism  for  altering 
the  position  of  the  body  in  the  water,  and  requires  a  small  amount  of 
flexibility,  being  moved  as  a  whole.  The  assumption  of  terestrial 
habits  necessitates  the  support  of  the  body  above  the  ground  and  its 
propulsion.  Hence  the  chiropterygium  must  have  a  firmer  skeleton, 
with  at  the  same  time  joints  for  motion  and  intrinsic  muscles  to  move  the 
parts  on  each  other.  The  chiropterygium  was  undoubtedly  derived 
from  the  fish  fin,  but  the  problem  of  how  the  change  was  made  has  not 
been  solved.     Only  paleontology  can  give  the  answer. 

There  are  two  views  as  to  the  origin  of  the  chiropterygium,  both  based  upon  the 
loss  of  certain  parts  and  the  persistence  of  others  in  a  modified  form.  One  view 
assumes  the  persistence  of  a  basal  as  the  framework  (humerus  or  radius)  of  the 


SKELETON. 


115 


upper  limb.  Two  proximal  radials  as  that  of  the  next  limb  segment,  while  the 
skeleton  of  ankle  and  foot  is  derived  from  a  corresponding  number  of  distal  radials 
on  the  anterior  side  of  the  fin.  The  'archipterygial  theory'  of  Gegenbaur  assumes 
an  appendage  like  that  of  Ceratodus  (the  *  archipterygium  *)  as  the  type  from 
which  all  legs  and  other  fins  have  been  derived,  by  a 
shortening  of  the  axis  and  a  loss  of  radials,  chiefly  on 
the  preaxial  side.  The  two  views  are  illustrated  in  the 
adjacent  sketches.  No  known  facts  of  either  embry- 
ology or  paleontology  throw  any  certain  light  on  the 
matter. 


»/ 


Cladoselache  (fig.  107)  and  the  lower  ganoids 
have  what  is  apparently  the  most  primitive  type 
of  fin  with  a  large  number  of  basalia  which 
support  a  large  number  of  radialia.  From  these, 
as  we  go  upward  in  the  scale,  there  is  a  reduction 
in  the  number  of  basalia,  either  by  disappear- 
ance or  fusion,  while  the  other  parts  are  variously 
modified.  Thus  in  recent  elasmobranchs  the 
characteristic  number  of  basalia  is  three  in  the 
pectoral,  two  in  the  pelvic  fin.  These  are  known, 
from  in  front  backward  as  the  pro-,  meso-,  and 
metapterygium,  the  middle  one  being  absent 


Fig.  123.  Fig.  124. 

Fig.  123. — ^Pelvic  fin  and  part  of  girdle  of  Ceratodus,  after  Davidoff.a,  axial  skeleton 
of  fin;  pil,  iliac  process;  pirn,  processus  impar;  r,  radialia. 

Fig.  124. — Skeleton  of  pectoral  fin  of  Xenacanthus,  after  Fritsch. 


from  the  hind  limb.  The  numerous  radials  are  jointed  transversely 
(fig.  109),  permitting  more  flexibility,  and  these  may  be  arranged 
entirely  on  one  side  of  the  basalia  (uniserial),  or  the  metapterygium 
may  be  prolonged  as  an  axis,  and  while  most  of  the  radialia  are 
on  the  preaxial  side,  some  may  occur  on  the  postaxial  side  (biserial) 
as  seen  in  the  carboniferous  shark,  Xenacanthus  (fig.  124).  In  the 
recent  species  the  skeleton  of  the  fin  is  continued  by  actinotrichia. 


ii6 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


In  the  male  elasmobranchs  the  pelvic  fin  is  divided  into  two  lobes, 
the  medial,  the  so-called  clasper  (mixipterygium)  being  the  longer  and 
narrower.  This  is  used  in  copulation  and  is  supported  by  thespecialized 
terminal  radialia  of  the  metapterygium. 

In  other  ganoids  and  in  teleosts  the  skeletal  parts  are  more  or  less 
ossified,  the  basalia  more  numerous  than  in  the  higher  elasmobranchs 
and  are  shortened  and  more  closely  associated  with  the  girdles,  while 

the  numerous  radii  form  most  of  the 
skeleton  of  the  fin  itself.  It  is  not  un- 
common for  the  anterior  element  of  the 
pectoral  fin  to  form  a  strong  defensive 
spine,  not  infrequently  connected  with  a 
poison  gland.  In  some  teleosts,  e.g., 
eels,  the  pelvic  fin  may  be  lacking. 
The  fins  of  the  dipnoi  are  easily  under- 
stood by  comparison  with  a  biserial  fin 
like  that  of  Xenacanthus  (fig.  1 24) .  The 
axial  part  has  been  elongated  and  in 
Ceratodus  it  bears  biscerial  radialia, 
while  in  Protopteriis  and  Lepidosiren 
only  the  axis  persists. 


Embryology  tells  little  as  to  the  primitive 


Fig.   125. — Cartilage  skeleton  of 
shoulder  girdle  and  left  pectoral  fin  of  ,.  .  r     ^       •  ^    ^  •  e      •       ^ 

larval  Polypterus,  after  Budgett,  bvf,  condition  of  the  ichthyopterygium,  for  in  the 
foramina  for  blood-vessels;  c,  cora-  precartilage   stage  the  condensation  of  mesen- 

^r^^.Lft'J;^'^':tt  r^^fT""'   •'^'''  chyme  for  the  skeleton  of  the  fin  forms  a  con- 
mesopteryguim;  WCT,  metapterygium;       -' 

pro,    protopteryigum;   r,  developing  tinuum  which  later   becomes  broken  into  the 
radialia;  s,  scapula.  separate  parts  (fig.  125). 

The  legs  (chiropterygia)  of  all  tetrapoda  are  essentially  alike  (fig. 
108).  Each  consists  of  several  regions,  comparable  in  detail  with  each 
other.  The  proximal  is  the  upper  arm  (brachium)  or  thigh  (femur) 
containing  a  single  bone,  the  humerus  or  femur  in  the  fore  and  hind 
limb  respectively.  The  next  region,  the  forearm  (antebrachium) 
or  shank  (crus),  contains  two  bones,  a  radius  or  tibia  on  the  preaxial 
and  an  ulna  or  fibula  on  the  postaxial  side.  Next  follows  the  podium, 
the  hand  (manus)  in  front,  the  foot  (pes)  behind,  each  consisting  of 
three  portions.  The  basal  podial  region,  the  wrist  (carpus)  or  ankle 
(tarsus)  consists  of  several  small  bones;  the  second  division  (metapo- 
dium)  is  the  palm  (metacarpus)  or  instep  (metatarsus)  and  lastly 
come  the  fingers  or  toes  (digits) ,  each  digit  consisting  of  several  bones. 


SKELETON. 


117 


the  phalanges.  These  separate  parts  are  included  in  the  accompany- 
ing table,  in  which  the  terms  given  to  the  separate  elements  of  the  wrist 
and  ankle  of  man  are  included. 


Fore  Limb 
Upper  arm  (Branchixim) 

Fore  arm  (Antebrachium 


Naviculare 

(Scaphoid) 
Lunatum 
Triquetrum 


Basi- 
podium 

Wrist 
(Carpus) 


Palm 

(Metapo- 

dium) 

Fingers  (Phalanges) 


Pisiforme 

Multangulum 

majus 
(Trapezium) 
Multangulum 

minus 
(Trapezoides) 
Capitatum 

Hamatum 


(arm) 

Humerus  =  Femur 
/  Radius = Tibia 

\  Ulna=Fibula 

Radiale  =Tibiale 


Intermedium  =  Intermedium 

Ulnare  =  Tibiale 

Centrale » + 2  =  Centrale '  +  2 


Carpale '  =  Tarsa  le ' 

Carpale2  =  Tarsale» 

Carpale'  =Tarsale' 
j  Carpale*  =  Tarsale*  \ 
\  Carpale*  =  Tarsale5  / 

Metacarpale  ^— *  =  Metatarsale ' 
Digits  *-**  =  Digits'-' 


Hind  Limb  (leg) 


Astragalus 
(Talus) 

Calcaneus 
Naviculare 

pedis 
(Scaphoid) 

Cuneiform* 


Cuneiform* 

Cuneiform' 
Cuboides 


Thigh 
\    Shank 
/    (Cms) 


Basi- 
podium 

Ankle 
(Tarsus) 


(Metapo- 
dium) 


Instep 
(Phalanges)  Toes 


The  basal  podial  region,  which  is  nearly  typical  in  some  reptiles, 
urodeles  and  man,  consists  of  three  rows  of  bones,  a  proximal  of  three 
bones,  a  radiale  or  tibiale  on  the  anterior  side,  an  ulnare  or  fibulare 
on  the  other,  and  an  intermedium  between  them.  The  distal  row 
consists  of  five  carpales  or  tarsales,  numbered  from  the  anterior  side. 
The  third  row  is  composed  of  one  or  two  centrales  between  the  other 
rows.  The  metapodials  and  the  digits,  also  numbered  from  one  to 
five,  have,  in  some  cases  special  names,  the  thumb  (digit  I)  being 
the  poUex,  the  corresponding  great  toe  being  the  hallux,  the  fifth 
digit  being  called  minimus. 

From  this  typical  condition  all  forms — legs,  arms,  wings — are 
derived  by  modification,  fusion  and  disappearance  of  parts.  The 
more  distal  a  part  the  more  variable  it  is;  reduction  takes  place  on  the 
margins  of  the  appendage,  the  axial  portions  being  the  last  to  disappear. 
Occasionally  in  various  groups  (amphibia,  mammals)  there  occur  what 


Il8        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

are  interpreted  as  rudimentary  additional  digits — prehallux,  prepol- 
lex,  postminimus — but  their  status  is  uncertain.  There  are  also  certain 
membrane  bones  developed  in  the  appendages  including  the  patella 
(knee-pan)  in  some  reptiles,  birds  and  many  mammals,  in  the  tendon 
that  passes  over  the  knee-joint,  the  fabellae  in  the  angle  of  the  knee  of 
a  few  mammals,  and  the  pisif orme  in  the  carpus  of  man  and  some  other 
mammals. 

In  the  ancestral  limb,  as  exemplified  in  the  urodeles,  the  basal 
joint  was  directed  horizontally  at  right  angles  to  the  axis  of  the  body,  but 
higher  in  the  scale  it  approaches  the  sagittal  plane  and  in  such  a  way 
that  the  angles  of  the  fore  and  hind  limbs  open  in  opposite  directions. 
Besides  there  is  frequently  a  torsion  of  the  bones 
of  the  forearm  (fig.  127)  or  shank  on  each  other. 
The  lower  amphibians  have  nearly  typical  legs, 
although,  as  in  Siren  and  Amphiuma,  they  may  be 
greatly  reduced,  while  in  some  stegocephals  and 


Fig.  126.  Fig.  127. 

Fig.  126. — 'Tarsus  of  Geotriton,.  after  Wiedersheim,  showing  the  arrangement  of  the 
metapodial  elements,  c,  centrale;  /,  fibulare;  F,  fibula,  i,  intermedium;  t,  tibiale;  T 
tibia;  1-5,  tarsales. 

Fig.  127. — Torsion  in  developing  human  arm,  after  Braus.  m,  r,  ulna  and  radius; 
dotted  line,  course  of  radial  nerve. 

the  gymnophiones  they  are  entirely  lacking.  In  the  anura  the  radius 
and  ulna  or  tibia  and  fibula  are  frequently  fused  and  the  tarsals 
elongated. 

The  most  marked  feature  of  the  reptilian  limb  is  the  occurrence  of 
an  intratarsal  joint,  the  motion  of  the  foot  upon  the  leg  being  largely 
between  the  two  rows  of  tarsal  bones,  instead  of  between  tarsus  and  the 
bones  of  the  shank  (fig.  128).  There  is  also  a  greater  range  of  form 
than  in  the  amphibia.  Limbs  are  lacking  in  snakes  and  some  lizards; 
on  the  other  hand  there  is  a  great  increase  in  the  number  of  phalanges, 
correlated  with  a  shortening  of  the  proximal  bones  in  the  plesiosaurs, 
which  reaches  its  extreme  in  the  ichthyosaurs  where  there  may  be  a 
hundred  phalanges  in  a  digit.     The  wings  of  the  pterodactyls  are  re- 


SKELETON. 


119 


markable  for  the  great  development  of  the  fifth  digit  (elongation  of  the 
phalanges)  as  a  support  for  the  wing;  the  other  digits  are  more  normal. 


Fig.  128. 


-Hind  leg  of  snapping  turtle   {Chelydra)  showing  intratarsal  joint  at  i.    h, 
humerus,  r,  radius;  u,  ulna;  I-V,  digits. 


The  wings  of  birds  (fig.  55)  are  even  more  modified.  Until  the 
carpus  is  reached  the  structure  is  approximately  normal,  but  the  carpal 
bones  are  greatly  reduced  by  fusion,  while  the  metacarpals  and  digits, 
extensively  modified,  number  only 
three.  Development  shows  that  the 
first  digit  is  entirely  lost  and  that  the 
fifth  metacarpal,  which  is  present  in 
the  embryo,  fuses  early  with  the 
fourth,  so  that  the  digital  formula 
is  II,  III,  IV.  There  is  also  an  ex- 
tensive fusion  of  the  bones  of  the 
tarsus  and  pes.  The  ankle-joint  is 
markedly  intratarsal,  the  basal  row 
of  tarsal  bones  fusing  with  the  tibia 
(the  fibula  is  reduced)  to  form  a 
*tibiotarsus,'  while  the  tarsales 
have  united  in  the  same  way  with 
the  fused  metatarsals,  forming  a 
*tarso-metatarsus'  (fig.  129). 
The  toes  are  rarely  more  than  four 
in  number,  the  first  apparently  lack- 
ing, and  as  a  rule  the  number  of 
phalanges  increases  from  two  in 
digit  II  to  five  in  digit  V.  Many 
birds  have  the  toes  reduced  to 
three  and  in  the  true  ostriches  to  two. 

In  the  mammals  the  limbs,  especially  the  fore  limbs,  exhibit  a  con- 
siderable range  of  modification.     Thus  in  the  primates  the  skeleton  is 


Fig.  129. — ^Foot  of  parrot  {Psittacus 
amazanicus),/,  femur;  fb,  fibula; p,  patella; 
tm,  tarsometatarsus;  tt,  tibiotarsus;  //-F, 
digits. 


I20        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

nearly  typical,  but  there  is  a  marked  power  of  rotation  of  the  foot  and 
especially  of  the  hand  by  the  motion  of  the  lower  end  of  the  radius 
around  the  ulna.  There  also  the  appendages  may  form  grasping 
organs,  both  features  being  found  to  a  less  extent  in  several  lower  groups. 
In  the  bats  digits  II  to  V  are  greatly  elongated  (either  metacarpals  or 
phalanges  may  be  lengthened)  to  support  the  wing,  the  first  digit  remain- 
ing normal.  In  the  whales  and  sirenians  the  basal  parts  of  the  fore 
limb  are  greatly  shortened,  while  there  is  a  multiplication  of  the  pha- 
langes, recalling  that  of  the  plesiosaurs.  The  hind  limb  is  entirely 
lacking  in  the  sirenians  and  some  of  the  whales;  in  other  whales  there 
are  two  vestigial  bones  ( Pfemur  and  tibia)  imbedded  in  the  muscles  of 
the  trunk. 

The  mammalian  humerus  is  frequently  perforated  by  a  (supra-  or 
entepicondylar)  foramen  passing  through  the  inner  lower  end,  a 
feature  found  elsewhere  only  in  some  theriomorphs.  In  many  un- 
gulates the  ulna  is  reduced  and  may  be  fused  with  the  radius;  elsewhere 
it  is  well  developed.  Even  where  reduced  it  always  bears  on  its  prox- 
imal end  a  strong  olecranon  process,  extending  beyond  the  elbow- 
joint  for  the  attachment  of  the  extensor  muscles  of  the  lower  limb. 
The  femur  bears  a  varying  number  (up  to  three)  of  prominences  or 
trochanters  for  the  attachment  of  muscles.  The  fibula  resembles  the 
ulna  in  its  tendency  to  reduction.  The  patella  (p.  ii8)  at  the  knee- 
joint  is  analogous  to  the  olecranon  process,  though  it  never  joins  the 
other  bones. 

The  details  of  the  modification  of  the  feet  cannot  be  described  here. 
The  ankle-joint  is  never  intratarsal  but  always  between  tarsal  and 
crural  bones.  There  is  considerable  variety  in  the  extent  to  which  the 
bones  of  the  feet  rest  upon  the  ground.  In  the  plantigrade  foot,  as  in 
the  bear  and  man,  the  sole  of  the  foot  includes  the  metapodial  bones;  in 
the  digitigrade  forms,  like  the  dog  and  cat,  the  sole  includes  only  the 
distal  phalanges,  while  in  unguligrades  (cow,  horse)  the  weight  of  the 
body  is  supported  on  the  hoofs  (p.  27)  developed  on  the  upper  (ante- 
rior) surface  of  the  distal  phalanges.  There  is  frequently  a  reduction 
of  the  digits,  reaching  its  extreme  in  the  horse  where  only  digit  III 
persists  in  a  functional  condition. 

THE  CCELOM  (BODY  CAVITIES). 

The  ccelom  includes  all  of  the  primitive  cavities,  right  and  left, 
enveloped   by  the   mesothelium  (p.   10).     With  the  division  of   the 


CCELOM. 


121 


walls  into  epimere,  mesomere  and  hypomere,  the  coelom  undergoes  a 
corresponding  division.  That  portion  in  the  epimere  is  divided  into 
a  series  of  cavities  in  the  myotomes  (myocoeles),  which  are  eventually 
obliterated  (p.  126);  the  portions  in  the  mesomere  persist  only  as  the 
lumina  of  the  excretory  organs  and  their  ducts,  described  under  the 
urogenital  system;  while  that  part  of  the  original  coelom  in  the  hypomere 
gives  rise  to  all  of  the  permanent  body  cavities  of  the  adult. 

The  hypomeres  gradually  descend  between  the  ectoderm  and  the 
entoderm  (fig.  130)  until  their  lower  margins  meet,  ventral  to  the  diges- 
tive tract.  In  this  way  the  latter 
becomes  surrounded  by  a  pair  of 
cavities,  the  splanchnocoeles  or  body 
cavities  of  the  adult.  Each  is 
bounded  by  epithelium,  the  tunica 
serosa,  in  which  an  outer  or  somatic 
wall  is  turned  toward  the  ectoderm, 
while  the  inner  or  splanchnic  wall 
adjoins  the  alimentary  canal.  Later, 
when  the  muscle  plates  extend  down- 
ward (fig.  135),  they  unite  ectoderm 
and  serosa  into  the  outer  body  wall, 
the  somatopleure,  while  the  invasion 
of  mesenchyme  unites  the  splanchnic 
serosa  with  the  entoderm  into  a  similar 
splanchnopleure. 

Mesenteries. — As  has  just  been 
stated  the  walls  of  the  two  ccelomic 
cavities  meet  below  the  digestive  tract,  thus  forming  a  double  membrane 
running  lengthwise  of  the  body  and  binding  the  alimentan^  canal  to  the 
ventral  body  wall.  This  membrane  is  called  the  ventral  mesentery. 
In  a  similar  way  the  splanchnic  walls  meet  above  the  digestive  tract 
forming  a  dorsal  mesentery.  These  mesenteries  are  eventually  more 
than  double  serosal  \valls,  since  mesenchyme  comes  in  between,  uniting 
them  and  affording  a  tissue  through  which  blood-vessels,  lymphatic 
vessels  and  nerves  can  reach  the  digestive  organs. 

For  convenience  of  reference  different  parts  of  these  mesenteries  have  received 
special  names,  according  to  the  organs  supported.  The  ventral  mesentery 
usually  almost  entirely  disappears,  only  a  small  portion  persisting  in  the  region 
of  the  liver,  the  mesohepar,  which,  in  the  ichthyopsida  may  carry  blood-vessels 


Fig.  130. — Diagram  of  early  meso- 
derm, showing  the  zones,  e,  h,  mm,  epi-, 
hypo-,  and  mesomeres,  the  walls  of  the 
coelom,  dm,  vm,  to  form  the  dorsal 
and  ventral  mesenteries.  A,  alimen- 
tary canal;  e,  ectoderm;  so,  sp,  soma- 
topleure and  splanchnopleure. 


122 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


to  that  organ.  The  dorsal  mesentery  is  usually  more  complete,  but  it  is 
interrupted  in  various  groups.  Its  regions  are  called  mesogaster,  mesentery 
proper,  mesocolon,  mesorectum,  etc.,  accordingly  as  they  support  stomach, 
small  intestine,  colon  and  rectum.  Except  in  the  cyclostomes  the  alimentary 
canal  is  bent  on  itself  and  the  bends  are  connected  by  similar  membranes,  here 
called  omenta.  These  also  have  special  names.  Thus  the  gastrohepatic 
omenttmi  (small  omentum)  connects  stomach  and  liver;  then  there  are  gastro- 
splenic,  doudeno-hepatic  omenta,  etc.,  while  in  mammals  there  is  a  great 
omentimi,  a  double  fold  of  mesogaster  and  mesocolon  which  connects  the  stomach 


Fig.  131. — Diagrammatic  section  of  a  vertebrate  to  show  the  relation  of  the  body  walls, 
etc.  av,  aorta;  c,  coelom;  e,  ectoderm;  ep,  epaxial  muscles;  ^,  gonads;  ^a,  haemal  arch;  /^/), 
hypaxial  miiscles;  i,  intestine;  mes,  mesentery;  w,  nephridium;  0,  omentum;  r,  rib;  jo, 
somatopleure;  sp^  splanchnopleure;  v,  vertebra. 

with  the  colon.  This  forms  a  large  sac,  the  bursa  omentalis,  which  opens  into 
the  rest  of  the  body  cavity  by  a  small  foramen  of  Winslow  (foramen  epiploicum) 
near  the  hinder  end  of  the  liver. 

Homologous  structures  are  formed  in  connection  with  other  organs.  Thus 
in  the  formation  of  the  heart  there  are  formed  temporary  membranes,  the  meso- 
cardia,  connecting  it  with  the  walls  of  the  pericardium;  while  in  the  mammals  a 
mediastinum,  between  the  two  pleural  cavities  binds  the  pericardium  to  the  ventral 
body  wall.  Frequently  the  reproductive  organs  project  so  far  into  the  body  cavity 
that  the  serosa  meets  behind  them,  forming  similar  supports,  mesovaria  for  the 
ovaries,  mesorchia  for  the  testes. 

The  primitive  body  cavity  extends  from  a  point  just  behind  the  head 
back  to  the  vent.  It  soon  becomes  divided  into  two  cavities.  Just 
in  front  of  the  liver  a  pair  of  blood-vessels,  the  Cuverian  ducts,  enter 
the  heart  from  the  sides.  These  arise  in  the  ventral  body  wall  but  soon 
ascend,  carrying  the  serosa  before  them.     In  this  way  they  form  a 


CCELOM. 


123 


transverse  partition,  the  septum  transversum,  attached  to  the  anterior 
wall  of  the  liver,  which  cuts  off  an  anterior  pericardial  cavity,  con- 
taining the  heart,  from  the  posterior  part  (metacoele)  of  the  body  cavity. 
In  many  lower  vertebrates  the  septum  is  not  complete,  but  one  or  more 
openings  (pericardio-peritoneal  canals)  connect  the  pericardium 
with  the  metacoele. 

In  the  mammals  a  second  partition,  the  diaphragm  (p.  135),  cuts 
off  another  pair  of  (pleural)  cavities  from  the  metacoele.  Traces  of 
similar  structures  occur  as  low  as  the  amphibia;  their  homology  with 
the  mammalian  diaphragm  is  not  always  certain,  but  in  some  cases  the 


Fig.  132. — Diagram  showing  the  relations  of  the  coelomic  cavities  (black)  in  A,  fishes, 
B,  amphibians  and  sauropsida;  and  C,  in  mammals;  H,  heart  in  pericardial  coelom; 
L,  liver;  P,  lungs  in  C  in  pleural  ccelom;  S,  septum  transversum;  D,  diaphragm. 

parts  concerned  have  the  same  nerve  supply.  The  development  of  the 
diaphragm  is  very  complicated  and  can  be  stated  here  only  in  outline. 
It  involves  in  part  the  septum  transversum,  in  part  is  a  new  formation. 
At  first  a  part  of  the  metacoele  extends  forward,  dorsal  to  the  pericardial 
cavity  and  alimentary  canal,  and  into  this  the  lungs  protrude  as  they 
are  developed.  Then  a  pair  of  muscular  folds  arise  from  the  dorsal 
surface  of  the  metacoele,  posterior  to  the  lungs;  these  grow  downward 
until  they  meet  the  septum  adjacent  to  the  attachment  of  the  liver, 
cutting  off  a  pair  of  pleural  cavities  containing  the  lungs,  from  the 
rest  of  the  metacoele,  now  known  as  the  peritoneal  cavity.  With 
increase  of  the  lungs  in  size  the  pleural  cavities  increase,  insinuating 
themselves  laterally  beween  the  pericardium  and  the  body  wall,  and 
eventually  reaching  the  ventral  side,  where  the  two  are  separated  by 
their  two  walls,  the  ventral  mediastinimi.  From  the  original  folds 
the  dorsal  muscles  of  the  diaphragm  are  derived;  the  ventral  come 
from  the  rectus  muscles  of  the  ventral  abdominal  wall.     The  dia- 


124 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


phragm  undergoes  many  shiftings  of  position  before  reaching  its  final 
place. 

The  tunica  serosa  lining  the  various  divisions  of  the  splanchnocoele  has  special 
names  in  each.  Thus  the  pericardial  and  pleural  cavities  are  lined  by  peri- 
carditun  and  pleura  respectively,  that  portion  of  the  pericardium  covering  the 
heart  being  sometimes  called  the  epicardium.  The  metacoele  or  peritoneal  cavity 
is  lined  by  the  peritoneum. 

The  metacoele  is  not  always  cut  off  completely  from  the  external 
world.  In  the  lower  vertebrates  the  urinary  ducts  frequently  open  into 
the  body  cavity  by  the  nephrostomes  (fig.  133),  and  in  these  and  even 
in  the  mammals  the  oviducts  of  the  female  connect  the  cavity  with  the 


'y'^y'y'^yy^^^^yyyy'yy^^^^^^':^^ 


Fig,  133. — Diagram  of  possible  connection  of  coelom  with  the  exterior,  modified  from 
Bles.  c,  ccelom;  c/,  cloaca;  g,  glomerulus  of  kidney;  i,  intestine;  «,  nephrostome;  pa, 
porus  abdominalis. 

exterior.  In  many  fishes  there  are  pori  abdominales  leading  from  the 
metacoele  to  the  outside  near  the  vent.  These  may  be  single  or  paired 
and  are  found  in  cyclostomes,  many  elasmobranchs  and  teleosts, 
ganoids,  and  dipnoi.  None  are  known  in  amphibia,  birds  or  mammals, 
but  in  turtles  and  crocodilians  so-called  peritoneal  canals  occur, 
usually  ending  blindly  in  chelonians,  but  emptying  into  the  cloaca 
in  the  crocodiles.  These  may  be  homologous  with  the  abdominal 
pores,  but  only  the  development  can  settle  the  question.  In  some 
fishes  the  pores  serve  for  the  escape  of  the  genital  products;  in  other 
animals  their  function  is  uncertain. 

THE  MUSCULAR  SYSTEM. 

Practically  all  motion  in  vertebrates  is  caused  by  muscles  arising  from 
the  mesoderm.  While  other  cells  may  have  a  certain  power  of  chang- 
ing shape,  the  muscle  cells  possess  this  in  a  marked  degree,  and  so 
that  they  may  cause  the  greatest  amount  of  motion  in  the  parts  to  which 
they  are  attached,  they  are  very  long,  stimulation  causing  them  to 
contract  in  length  and  at  the  same  time  to  increase  in  diameter. 


MUSCULAR   SYSTEM.  I25 

There  are  two  kinds  of  muscles  which  differ  in  origin,  histological 
appearance,  physiological  action  and  distribution.  The  smooth 
muscles,  the  appearance  of  which  has  been  described  (p.  20),  arise 
from  the  mesenchyme  and  are  not  under  control  of  the  will,  but  are 
innervated  by  the  sympathetic  nerv^ous  system.  Their  action  is  much 
slower  than  that  of  the  other  type.  They  are  found  in  the  skin,  in  the 
walls  of  blood-vessels  and  of  the  alimentary  canal,  and  in  the  urogenital 
system.  Occasionally  they  occur  as  isolated  fibres,  but  frequently 
they   form   sheets   or  bands,   sometimes   of   considerable   thickness. 

In  the  alimentary  tract  they  are  arranged  in  two  layers  in  the  straight 
parts  of  the  tube,  an  outer  layer  of  fibres  which  run  longitudinally, 
and  inside  this  a  layer  of  circular  muscles.  In  enlargements  of  the 
tube  this  regularity  is  interrupted  and  the  course  of  the  fibres  is  more 
irregular.  The  circular  muscles,  by  their  contraction,  lessen  the  diam- 
eter of  the  canal,  at  the  same  time  causing  it  to  elongate,  while  the 
longitudinal  fibres  shorten  it  and  cause  it  to  increase  in  diameter.  In 
the  blood-vessels  there  are  only  circular  fibres,  the  enlargement  of  the 
lumen  being  caused  by  the  internal  blood  pressure. 

The  striped  muscles  are  derived  from  the  walls  of  the  coelom  and 
hence  are  of  mesothelial  origin.  Excepting  those  of  the  heart  (to  be 
mentioned  below)  and  some  of  those  at  the  anterior  end  of  the  alimentary 
canal,  they  are  under  control  of  the  will  and  are  supplied  by  the  motor 
nerves  of  the  brain  and  spinal  cord.  They  are  also  able  to  contract, 
more  rapidly  than  the  smooth  muscles.  The  striped  muscles  make  up 
the  great  mass,  of  the  musculature — the  'flesh' — of  the  body.  They 
occur  in  the  body  walls,  organs  of  locomotion,  the  head,  diaphragm 
and  the  anterior  part  of  the  digestive  canal. 

The  voluntary  muscles  are  derived  in  part  from  the  somites  (myo- 
tomes), in  part  from  the  lateral  plates,  the  latter  furnishing  the  vis- 
ceral muscles,  including  those  of  the  head  (except  the  eye  muscles  and 
the  sternohyoid  and  its  derivatives  in  the  higher  vertebrates)  and  those 
of  the  heart.  The  heart  muscles,  the  development  of  which  is  traced  in 
the  account  of  the  circulatory  system,  differ  from  the  other  striped 
muscles  in  the  uninucleate  condition  of  their  short  and  usually  branched 
cells,  while,  physiologically,  they  are  involuntary  in  character. 

THE  PARIETAL  MUSCLES. 

After  the  myotomes  are  cut  off  from  the  rest  of  the  coelomic  walls 
(p.  14)  each  consists  of  a  closed  sac,  containing  a  part  of  the  coelom 


126 


COMPARATIVE   MORPHOLGY   OF   VERTEBRATES. 


(myocoele)  and  an  inner  (splanchnic)  and  an  outer  (somatic)  wall 
The  cells  of  the  splanchnic  wall  rapidly  increase  in  number  and  size, 
thus  tending  to  obliterate  the  myocoele.  At  the  same  time  they  be- 
come rearranged,  so  that,  instead  of  forming  a  cubical  or  columnar 
epithelium,  they  have  their  long  axis  parallel  to  the  long  axis  of  the  body 


Fig.  134. — Myotomes  of  Amhlystoma  developing  into  muscle  fibres,     ec,  ectoderm;  wy, 
myocoele;  ms,  mesenchyme;  so,  somatic  layer  which  will  form  corium. 

(fig.  134),  each  becoming  multinucleate.  Gradually  the  mass  of  the 
protoplasm  becomes  converted  into  contractile  substance  and  the  cell 
is  converted  into  a  muscle  fibre,  the  nuclei  being  in  the  interior  in  the 
lower  vertebrates,  on  the  surface  of  the  fibres  in  the  mammals.  In  this 
way  the  splanchnic  wall  of  each  myotome  is  converted  into  a  muscle; 


Fig.  135, — ^Diagram  of  descending  myotomes,  c,  coelom;  g,  gonad;  m,  splanchnic  wall 
of  myotome  developing  into  muscles;  mc,  myocoele;  p,  peritoneum;  pd,  pronephric  duct; 
so,  somatic  wall  of  myotome;  v,  ventral  border  of  myotome. 

hence  there  are  as  many  pairs  of  these  primitive  muscles  as  there  were 
of  myotomes.  The  somatic  wall  of  the  myotome  does  not  participate 
in  the  muscle  formation,  but  is  gradually  changed  into  mesenchyme 
and  eventually  gives  rise  to  the  corium  of  the  skin.  Mesenchyme  also 
invades  the  spaces  between  the  successive  myotomes,  develops  into 


MUSCULAR   SYSTEM. 


127 


fibrous  connective  tissue,  and  forms  the  ligamentous  connections 
(myosepta,  myocommata)  between  the  muscles  of  a  side.  This 
primitive  condition  is  readily  recognized  in  the  trunk  and  tail  of  the 
lower  vertebrates,  and  even  in  the  adults  of  the  more  modified  birds 
and  mammals  the  original  segmentation  can  be  traced  in  the  inter- 
costal and  rectus  abdominis  muscles.  At  first  the  myotomes  lie 
at  about  the  level  of  the  notochord  and  spinal  cord,  but  with  growth 
they  extend  upward  and  to  a  greater  extent  downward,  insinuating 
themselves  between  the  skin  and  the  w^alls  of  the  ccelom  and  thus 


Fig.  136. — Head  of  embryo  dogfish  {Acanthias)  seen  as  a  transparent  object,  showing 
the  preotic  mesodermal  somites,  with  dotted  outlines,  as  a,  i,  2,  and  3.     b^-b*,  gill  clefts, 
the  fifth  not  yet  open;  e,  eye;  oc,  otic  capsule;  p,  epiphysial  outgrowth;  s,  spirade;  V,  tri 
geminal,  VII,  facial-acustic;  IX,  glossopharyngeal;  X,  vagus  nerves. 


forming  part  of  the  somatopleure.  The  downward  growth  continues 
imtil  the  muscles  of  the  two  sides  all  but  meet  in  the  mid-ventral  line, 
the  intervening  space  being  occupied  by  connective  tissue,  the  linea 
alba  of  the  adult. 

In  the  fishes  the  trunk  and  tail  muscles  formed  in  this  way  become 
divided  horizontally  into  dorsal  and  ventral  portions,  the  epaxial  and 
hypaxial  muscles,  the  line  of  division  which  follows  more  or  less 
closely  the  lateral  line,  bein  g  marked  by  a  partition  of  connective  tissue 
already  mentioned  (figs.  30,  131).  These  plates  of  muscle  do  not  retain 
their  flat  ends  in  the  adult,  but  one  end  becomes  conical  and  fits  into  a 
corresponding  hollow  in  the  next  plate.     In  the  tail  of  the  amphibia 


128 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES, 


epaxial  and  hypaxial  muscles  are  clearly  recognizable,  but  farther 
forward  the  hypaxials  are  greatly  reduced,  and  in  the  amniotes  the 
reduction  is  carried  so  far  that  the  hypaxial  muscles,  greatly  modified, 
can  only  be  recognized  in  the  cervical  and  pelvic  regions. 

In  the  head  the  developmental  conditions  are  more  complicated 
than  in  the  trunk,  our  information  being  most  complete  with  regard  to 
the  ichthyopsida.  Here,  in  the  region  which  develops  into  the  head, 
ten  coelomic  pouches  are  developed  (in  amniotes  the  number  is  appar- 
ently twelve) .  These  are  known  by  number,  except  that  the  most  anter- 
ior, which  was  not  known  when  the  numbers  were  applied  is  called  A. 
These  coelomic  cavities  (also  known  as  head  cavities)  differ  from  the 

myotomes  farther  back  in  having  no  undivi- 
ded portion  of  the  coelom  below,  correspond- 
ing to  the  hypomeral  zone,  a  difference  possi- 
bly due  to  the  existence  of  visceral  clefts  in 
this  region  (fig.  136). 

Four  of  these  cavities  lie  in  front  of  the  ear. 
Of  these  A  disappears  completely,  its  cells 
joining  the  mesenchyme,  while  the  other  three 
give  rise  to  the  'eye  muscles'  which  move 
the  eye-ball.  Without  going  into  all  of  the 
details,  i,  which  lies  in  front  of  the  mouth, 
gives  rise  to  the  superior  oblique  muscle ;  2, 
which  lies  in  the  region  of  the  jaws,  forms  four 
muscles,  the  inferior  oblique  and  three  of 
the  rectus  muscles  (in  some  forms  also  a 
retractor  bulbi),  while  2,  in  the  hyoid  region,  develops  the  external 
rectus.  This  method  of  origin  explains  the  distribution  of  the  eye- 
muscle  nerves  to  be  described  later,  each  nerve  supplying  only  the 
derivatives  of  a  single  myotome.  Several  of  the  other  head  myo- 
tomes disappear  in  development,  while  the  posterior  form  the  so-called 
hypoglossal  musculature  (fig.  138). 

In  the  above  account  there  is  given  merely  the  origin  of  the  con- 
tractile tissue  of  the  muscles.  To  this  other  parts  of  connective  tissue 
are  added.  Mesenchyme  cells  invade  the  masses  of  muscle  fibres, 
forming  envelopes  (perimysium)  which  bind  the  fibres  into  bundles 
(fasciculi)  which,  in  turn,  are  united  by  other  envelopes,  the  fascia. 
These  connective-tissue  envelopes  are  extended  beyond  the  contractile 
tissue  and  form  the  cords  or  tendons  by  which  the  muscle  is  attached  to 


Fig.  137. — Diagram  of 
the  eye  muscles  of  the  right 
eye,  seen  from  the  medial 
side,  er,  external  rectus ;  ?/>, 
inferior  rectus;  io,  inferior 
obhque;  itr,  internal  rectus; 
so,  superior  oblique ;  sr,  supe- 
rior rectus;  ///,  coulomotor; 
IV,  trochlearis;  VI,  abducens 
nerves. 


MUSCULAR   SYSTEM.  1 29 

other  parts.  One  point  of  attachment,  the  origin,  is  fixed,  that 
to  the  part  to  be  moved  is  called  the  insertion.  Tendons  may  be  long  and 
slender,  allowing  the  muscle  to  lie  in  or  near  the  trunk,  while  the  part 
to  be  moved  is  in  the  appendage.  Again  they  may  form  broad  flat 
sheets  (aponeuroses),  and  these  may  occur  not  only  at  the  ends  but  in 
the  middle  of  a  muscle.  Not  infrequently  parts  of  tendons  may  ossify, 
as  in  the  patella  or  in  the  'drum-stick'  of  the  turkey.  Small  rounded 
ossifications  of  this  kind  are  called  sesamoid  bones.  In  a  few  cases 
the  parietal  muscles  are  without  attachment,  but  form  rings  which  are 
used  to  diminish  the  size  of  an  opening  (sphincter  muscles). 

Muscles  vary  greatly  in  shape.  They  are  usually  short  and  flat  in  the  trunk, 
prismatic  or  spindle-shaped  in  the  appendages.  They  may  be  simple  or  they 
may  have  several  'heads'  or  points  of  origin  (biceps,  triceps,  etc.),  or  several 
points  of  insertion  as  in  pinnate  or  serrate  muscles.  Again,  there  may  be  two 
or  more  contractile  portions  (bellies)  in  a  muscle,  separated  by  a  tendon  or 
aponeurosis. 

Usually  muscles  are  arranged  in  antagonistic  groups,  the  action  of  one  being 
the  opposite  of  its  antagonist.  Thus  there  are  flexors  to  bend  a  limb,  extensors  to 
straighten  it;  elevators  to  close  the  jaw,  depressors  to  open  it;  sphincters  working 
against  dilators,  etc. 

Only  a  few  points  in  the  progressive  modifications  of  the  primitive 
musculature  described  above  can  be  mentioned  here,  partly  from  lack  of 
space,  partly  from  deficient  knowledge.  There  are  great  difl&culties  in 
tracing  exact  homologies  through  the  different  groups  of  vertebrates, 
on  account  of  their  very  different  fimctions  in  the  separate  classes  and 
their  great  variability,  even  in  the  same  family.  The  best  test  of 
homology  is  nerve  supply,  every  muscle  derived  from  any  one  myotome 
being  innervated  by  branches  of  the  nerve  originally  connected  with 
the  segment,  as  is  beautifully  illustrated  in  the  case  of  the  eye  muscles 
as  mentioned  above.  Next  in  importance  are  origin  and  insertion  of 
the  muscles,  while  the  work  done  by  the  muscles  is  of  little  value. 
Differentiations  from  the  primitive  condition  may  take  place  in  various 
ways.  A  single  muscle  may  split  into  layers  or  it  may  divide  longi- 
tudinally into  two  or  more  distinct  muscles,  or  transversely  into  two 
successive  portions.  On  the  other  hand,  two  muscles,  different  in 
origin,  may  fuse,  while  with  loss  of  function  of  a  part,  its  muscles  may 
degenerate  or  entirely  disappear.  Muscles  may  wander  far  from  their 
point  of  ontogenetic  origin  and  become  connected  with  parts  widely 
remote,  a  condition  strikingly  illustrated  in  the  facial  muscles  of  the 
9 


I30 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


higher  mammals,  where  nerve  supply  still  shows  the  original  history. 
In  the  ichthyopsida  the  trunk  muscles  clearly  show  their  myotomic 
origin,  but  even  here  there  are  tendencies  to  division  and  specialization. 
The  ventral  muscles  on  either  side  of  the  body  cavity  of  the  amphibia 
(fig.  140)  are  divided  into  a  lateral  oblique  and  a  medial  rectus  sys- 
tem, the  rectus  muscles  of  the  two  sides  being  separated  by  the  linea 
alba  already  referred  to.  The  rectus  muscles,  in  turn,  become  divided 
into  successive  groups,  a  rectus  abdominis  in  the  abdominal  region, 
extending  from  the  pelvis  to  the  sternum;  a  sternohyoid  from  the 


hyp.n 


Fig.  138. — Diagram  of  muscle  segments  in  head  of  embryo  vertebrate,  based  upon  a 
shark,  after  Neal.  The  anterior  myotomes  tend  to  divide  into  dorsal  and  ventral  moieties; 
persistent  myotomes  lined,  transient  writh  broken  lines;  central  nervous  system  dotted, 
nerves  black,  a,  premandibular  somite;  ah,  abducens;  nerve,  hyp,  hypoglossal  musculature; 
hypn,  hypoglossal  nerves;  om,  oculomotor  nerve;  sp,  spiracle;  1-6,  first  six  somites  (4,  5,  6, 
functional  in  Petromyzon);  I-VIII  neuromeres. 

Sternum  to  the  hyoid  bone,  and  a  geniohyoid  from  the  hyoid  to  the  tip 
of  the  lower  jaw.  The  oblique  region  is  also  divided  into  three  layers 
(obliques  and  transversus)  characterized  by  the  direction  of  the  fibres. 
In  the  higher  vertebrates,  with  the  appearance  of  well  developed  ribs, 
the  oblique  muscles  furnish  the  two  layers  of  intercostal  muscles, 
extending  from  rib  to  rib,  and  in  front  of  the  ribs  they  form  the  scalene 
muscles,  extending  from  the  ribs  along  the  side  of  the  neck,  and  the 
sternocleidomastoid,  from  the  breast  bone  and  clavicle  to  the  skull. 
In  the  non-placental  mammals  a  strong  pyramidalis  muscle  extends, 
ventral  to  the  rectus,  from  the  inner  side  of  the  marsupial  bones  to  the 
sternum,  but  disappears  with  these  bones. 

The  dorsal  muscles  are  more  conservative,  undergo  less  modifica- 
tion than  those  just  mentioned,  and  always  show,  more  or  less  clearly, 
their  metameric  nature.  They  become  connected  with  various  parts 
of  the  vertebrae  and  with  the  ribs,  and  are  correspondingly  divided  into 


MUSCULAR   SYSTEM.  I3I 

different  groups.  Thus  the  spinales  connect  the  spinous  processes,  the 
transversales  the  transverse  processes  of  the  successive  vertebrae, 
while  the  transverse -spinales  extend  from  the  transverse  process  of 
one  vertebra  to  the  spinous  process  of  the  next.  In  the  higher  verte- 
brates the  anterior  spinaUs,  connecting  the  first  vertebra  with  the  skull, 
is  divided  into  several  rectus  capitis  muscles.  The  longissimus 
dor  si  group  extends  from  the  pelvis  to  the  head,  lying  on  either  side  in 
the  angle  between  spinous  and  transverse  processes.  It  may  be  differen- 
tiated into  separate  muscles — a  longissimus  dorsi  proper  in  the  lumbar 
region,  an  ileo-costalis  inserted  on  the  dorsal  part  of  the  ribs,  and  a 
longissimus  capitis  along  the  side  of  the  neck  to  the  temporal  region 
of  the  skull. 

The  muscles  which  move  the  appendages  are  divided  into  the 
intrinsic,  which  are  located  in  the  limb  itself  and  have  their  origin 
either  from  the  bones  of  the  Umb  or  from  the  supporting  girdle,  and  the 
extrinsic,  which  have  their  origin  on  the  trunk  and  are  inserted  on  the 
girdle  or  the  base  of  the  limb.     The  latter  move  the  limb  as  a  whole, 


Fig.  139. — Budding  of  muscles  of  appendage  from  myotomes  in  Pristiunis^  after  Rabl 
6,  muscle  buds;  wy,  myotomes. 

while  the  intrinsic  bend  the  limb  on  itself.  As  would  be  expected  from 
the  motions  of  the  fins,  the  intrinsic  muscles  are  hardly  noticeable  in  the 
fishes,  the  various  movements  being  accomplished  by  the  extrinsic 
group.  These  latter  are  divided  into  protractors  which  draw  the 
member  forward;  retractors  which  pull  it  back  against  the  body; 
levators  which  lift  it  and  depressors  which  pull  it  down. 

In  those  vertebrates  which  are  sufficiently  known  the  first  traces  of  the  develop- 
ment of  the  musculature  of  the  appendages  are  the  appearance  of  two  buds  (fig. 
139)  from  the  ventral  border  of  a  varpng  number  of  myotomes  in  the  region  of  the 
developing  limb.  These  buds  proliferate  cords  of  cells  which  soon  lose  their 
distinctness  and  form  a  blastema  from  which  the  intrinsic  muscles  arise,  the 
definitive  muscles  being  innervated  by  as  many  spinal  nerves  as  there  are  contribut- 
ing myotomes.     The  extrinsic  muscles  arise  directly  from  the  myotomes. 


132         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

With  the  development  of  the  paired  appendages  into  organs  for  the 
support  of  the  body  (tetrapoda)  the  skeleton  of  the  leg  is  converted  into 
a  series  of  levers,  and  the  intrinsic  muscles  are  correspondingly  dif- 
ferentiated and  developed.  Details  cannot  be  given  here  as  there  are 
so  many  modifications,  but  they  may  be  grouped  as  flexors,  which 
bend  the  limb  or  its  parts;  extensors  which  straighten  it,  and  rotators 
which  turn  it  on  its  axis.  These  undergo  the  most  modification  in  the 
peripheral  regions,  the  muscles  of  the  upper  arm  and  thigh  being  more 
constant  in  character  and  position.  Even  more  constant  are  the  ex- 
trinsic muscles,  which  may  be  grouped  as  in  fishes.     Most  prominent 


^^C" 


Fig.  140. — Superficial  muscles  of  anterior  part  of  Salamandra  ntaculata,  after  Fiir- 
bringer.  a,  anconeus;  bi,  humero-branchialis  inferior  (biceps);  bs,  levator  scapulae;  cmc, 
cucularis;  dtr,  dorso-trachealis;  dg,  digastric;  ds,  dorsalis  scapulae;  eo,  external  oblique; 
Id,  latissimus  dorsi;  m,  petro-tympano-maxillaris  (masseter);  mh,  mylohyoid;  pc,  pectoralis; 
ph,  procoraco-humeralis;  ra,  rectus  abdominis;  spc,  supracoracoid. 

of  the  levators  of  the  fore  limb  are  the  trapezius  and  levator  scapulae 
muscles,  while  the  pectoralis  and  serratus  anterior  act  as  depressors; 
the  sternocleidomastoid  and  the  levator  scapulae  anterior  act  as 
protractors,  the  pectoralis  minor  and  the  latissimus  dorsi  are  their 
antagonists.  In  the  pelvic  region  the  extrinsic  muscles  are  less  dif- 
ferentiated in  function.  The  pectineus  and  adductors  act  as  pro- 
tractors, the  pyriformis  counteracts  them;  the  limb  is  drawn  toward 
the  middle  line  by  a  pubofemoralis,  while  the  gluteus  muscle  acts  as 
a  retractor  and  elevator. 

THE  VISCERAL  MUSCLES. 

In  the  gill-bearing  vertebrates  a  special  system  of  muscles  is  devel- 
oped in  connection  with  the  visceral  arches,  which  have  to  open  and 
close  the  visceral  clefts,  including  the  mouth.  With  the  loss  of  the  gills 
some  of  these  muscles  are  lost  while  others  become  changed  in  function, 
several  retaining  their  connection  with  the  hyoid.  These  visceral 
muscles  may  be  divided  into  two  sets  according  as  they  are  derived 


MUSCULAR   SYSTEM. 


^33 


from  muscles  which  originally  ran  in  a  transverse  (circular)  or  in  a 
longitudinal  direction. 

To  the  first  category  belong  the  epibranchial  muscles,  the  sub- 
spinales  and  interbasales,  which  lie  in  the  dorsal  part  of  the  branchial 
region,  while  the  coraco-arcuales  are  in  the  ventral  or  hypobranchial 
half.  The  most  anterior  of  the  circular  group  are  those  which  open 
(digastric  or  depressor  mandibulae)  or  close  (adductors)  the  mouth, 
and  the  mylohyoid  which  extends  between  the  two  rami  of  the  lower 


Fig.  141. — Dorsal  and  ventral  head  muscles  of  the  skate  {Raid),  after  Marion;  the  dorsal 
muscles  more  deeply  dissected  on  the  left  side,  the  ventral  on  the  right  and,  lateral  man- 
dibular adductors;  amm,  medial  mandibular  adductors;  csd,  csv,  dorsal  and  ventral  con- 
strictors; cm,  coraco-mandibularis;  chy,  coraco  hyoideus;  chm,  coraco-hyomandibularis; 
cbr,  coraco-brachialis;  cac,  common  coraco- arcual;  intbr,  interbranchials;  Us,  superior 
labial  levators;  Imi,  levator  of  lower  jaw;  Ihm,  hyomandibular  levator;  /r,  levator  of  rostrum; 
<r,  trapezius;  VII,  seventh  nerve;  dm,  depressor  mandibulae  (digastric). 

jaw.  Usually  there  are  several  adductors,  known  as  masseter,  tem- 
poralis, pterygoideus,  accordingly  as  they  have  their  origin  from 
different  parts  of  the  skull.  The  longitudinal  muscles  are  largely  con- 
fined to  small  slips  which  pass  from  one  arch  to  the  next.  In  the 
amphibians  these  various  muscles  undergo  considerable  differentia- 
tion, while  in  the  amniotes  this  is  in  part  carried  farther,  in  part  is  re- 
duced on  account  of  the  loss  of  branchial  respiration  and  the  degenera- 
tion of  the  parts  connected  mth  it.  Hence  the  most  noticeable  of  the 
visceral  muscles  are  those  connected  with  opening  and  closing  the 
mouth. 


134 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

THE  DERMAL  MUSCLES. 


The  muscles  already  mentioned  are  connected  with  the  skeleton, 
but  in  the  higher  vertebrates  a  dermal  musculature  appears  in  which 
the  muscles  are  inserted  in  the  skin,  although  they  are  derived  from  the 
skeletal  muscles.  This  system  is  poorly  developed  in  the  amphibia, 
and  increases  in  the  reptiles  and  birds,  where  it  serves  to  move  the 
scales,  scutes  and  feathers.  It  is  especially  noticeable  in  the  snakes, 
where  it  is  largely  concerned  in  the  movement  of  the  scutes  in 
creeping. 

The  system  acquires  its  greatest  development  in  mammals.  In  the 
marsupials,  for  instance,  there  is  an  extensive  dermal  musculature,  the 
panniculus  carnosus,  covering  a  large  part  of  the  body  and  the  ap- 
pendages.    It  is  by  means  of  this  that  various  mammals  twitch  the 


Fig.  142. — ^Principal  dermal  muscles  of  head  of  man.  aa,  as,  auriculares  anterior  and 
superior;/,  frontalis;  m,  ma,sseter;  oc,  occipitalis;  00,  orbicularis  oris;  op,  orbicularis  pal- 
pebrarum; jfrw,  platysma  myoides;  s,  sternocleidomastoid;  t,  trapezius. 

skin  to  dislodge  insects,  etc.,  while  armadillos  and  hedgehogs  roll  them- 
selves into  a  ball  by  means  of  a  part  of  the  layer.  In  the  primates  the 
dermal  muscles  are  restricted  to  the  neck  (platysma  myoides)  and  the 
head,  all  parts  of  them  being  supplied  by  the  facial  nerve  belonging 
primitively  to  the  hyoid  region.  The  platysma  extends  forward  from 
the  neck  and  by  growth  and  division  gives  rise  to  the  muscles  of  ex- 
pression— the  orbiculares  which  close  the  lips  and  eyelids,  the  muscles 


MUSCULAR   SYSTEM.  I35 

which  lift  lips,  nose  and  lids  and  those  which  move  the  ears — muscles 
which  as  a  whole  have  their  highest  development  in  man  (fig.  142). 

THE  DIAPHRAGM. 

The  diaphragm  is  a  transverse  voluntary  muscle  which  crosses  the 
body  cavity  of  the  mammals  just  behind  the  pericardium  and  lungs. 
Its  muscles  are  in  part  derived  from  those  of  the  back,  in  part  from  the 
rectus  muscles  of  the  lower  surface.  Various  attempts  have  been  made 
to  recognize  similar  muscles  in  the  lower  vertebrates,  in  some  cases 
with  considerable  success.  Its  development  is  outlined  in  the  section 
on  the  coelom  (p.  123).  The  diaphragm  is  dome-shaped  and  is  attached 
to  the  vertebral  column  and  to  the  ribs.  It  is  traversed  by  the 
oesophagus  and  the  large  arterial  and  venous  trunks.  In  some  verte- 
brates the  muscular  portion  is  confined  to  the  margin,  the  centre  being 
membranous;  in  others  the  muscle  fibres  extend  across  it.  Contrac- 
tion of  the  muscles  flatten  it,  thus  enlarging  the  pleural  cavities  and 
drawing  air  into  the  lungs,  thus  aiding  in  respiration.  It  is  supplied  by 
the  phrenic  nerve. 

ELECTRICAL  ORGANS. 

It  is  well  known  that  the  contraction  of  a  muscle  causes  the  dis- 
charge of  a  minute  amount  of  electrical  energy,  so  it  is  not  surprising 
that  in  certain  cases  muscles  are  modified  into  electrical  organs.  The 
known  cases  occur  only  in  elasmobranchs  and  teleosts.  The  discharge 
is  w^eak  in  most  species,  but  is  strong  in  Torpedo  and  Gymnotus.  In  all 
but  Malapterurus  the  electrical  organs  are  clearly  modified  muscles,  situ- 
ated in  the  head  in  Torpedo  and  Asiroscopus,  in  the  trunk  of  Gymnotus , 
and  in  the  tail  of  Raia,  the  nerve  supply  being  correspondingly  varied. 
Thus  in  Torpedo  the  seventh,  ninth  and  tenth  cranial  nerves  are  con- 
cerned, while  in  Gymnotus  and  the  skates  the  supply  is  from  the  spinal 
nerves.  Malapterurus  is  peculiar  in  that  the  organ  is  in  the  integu- 
ment and  has  been  supposed  by  some  to  arise  from  modified  glands. 
It  is  more  probable  that  here  as  elsewhere  it  is  derived  from  the  muscles, 
as  the  organ  is  under  control  of  the  will;  the  development  has  yet  to  be 
studied.  This  diversity  of  origin  clearly  shows  that  the  electrical 
functions  have  been  separately  acquired  in  the  different  species. 

The  organs  are  composed  of  a  large  number  of  electrical  plates 


136 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


(electroplaxes)  arranged  at  right  angles  to  the  axis  of  the  primitive 
muscle,  each  derived,  where  the  history  has  been  traced  {Torpedo, 
Rata),  from  a  primitive  muscle  cell.  In  the  typical  condition  each 
plate  consists  of  an  outer  electric  layer,  differentiated  into  a  nervous  side 
and  a  so-called  nutritive  side,  with  a  middle  striated  layer  between  them, 
the  latter  in  a  few  cases  being  weakly  developed  or  absent.  Nervous 
stimulation  is  always  by  motor  roots  leading  to  the  nervous  layer,  the 
connexion  corresponding  to  the  nerve-end  of  a  muscle  cell.     Numbers 


Fig.  143. — Head  of  Astroscopus  y-grcecum,  after  Dahlgren  and  Silvester.  The  dotted 
line  on  right  shows  extent  of  electric  organ,  on  the  left  the  eye- muscles,  and  nerves  as  forced 
out  of  place  by  the  electric  organ,  ah,  abducens;  cil,  ciliary  nerve;  e,  eye;  en,  electric  nerve; 
n,  xiaris;  olf,  olfactory  nerve;  om,  oculomotor;  op,  optic  nerve;  re,  rif,  rini,  rs,  external,  in- 
ferior, internal  and  superior  rectus  muscles;  rp,  palatine  nerve;  so,  superior  oblique  muscle; 
//,  trigeminal  facial  nerve. 


of  these  electroplaxes  are  included  in  a  connective-tissue  compartment 
with  a  gelatinous  substance  between  them  and  all  with  their  nervous 
layer  turned  in  the  same  direction. 

In  Torpedo  the  organ  apparently  is  derived  from  part  of  the  jaw 
muscles  and  the  prisms  of  plates  are  arranged  vertically.  In  Astro- 
scopus (fig.  143)  it  is  supposed  that  the  tissue  comes  from  one  of  the  eye 
muscles,  while  in  Gymnotus  the  ventral  trunk  muscles  are  concerned 
and  the  columns  of  electroplaxes  are  horizontal.  In  the  same  fish  the 
discharge  is  always  in  the  same  direction,  e.g,^  in  Torpedo  from  below 
upward. 


NERVOUS   SYSTEM.  I37 

THE  NERVOUS  SYSTEM. 

Nervous  and  sensory  structures  are  closely  related  to  each  other, 

►  and  their  distinction  in  the  higher  animals  is  the  result  of  differentiation 

among  cells  which  were  originally  both  nervous  and  sensory  in  character, 

and  it  is  in  this  broader  sense  that  the  term  nervous  structures  is  used 

in  these  introductory  paragraphs. 

The  nervous  system  primarily  has  to  inform  the  animal  of  the  con- 
ditions, good  and  bad,  in  the  environment,  to  correlate  this  information 
and  to  regulate  the  motions  so  that  advantage  may  be  had  of  this  knowl- 
edge. These  facts  have  determined  several  features  of  the  nervous 
system.  Thus  they  have  determined  its  origin  in  the  ectoderm,  the 
outer  layer  of  the  body,  which  comes  into  relation  with  the  external 
world.  Since  this  information  has  to  be  carried  to  internal  parts,  con- 
ducting tracts  or  nerves  have  arisen,  while  the  correlating  fimction 
has  been  localized  in  the  body  of  the  cells  where  incoming  and  out- 
going tracts  meet. 

Most  important  of  the  primitive  functions  was  the  determination 
of  the  character  of  the  food,  which  would  lead  to  the  greater  aggregation 
of  the  nervous  tissue  around  the  mouth.  As  we  have  seen  (p.  ii) 
the  anlage  of  the  central  nervous  system  of  the  vertebrates  occupies 
such  a  position. aroimd  the  blastopore,  or  mouth  of  the  gastrula,  in  the 
form  of  the  neural  plate.  As  the  external  surface  of  the  body  is  naost 
exposed  to  injury,  the  nervous  structures,  with  the  closure  of  the  blasto- 
pore, have  been  protected  by  removal  to  a  deeper  position,  through  the 
rolling  of  the  plate  into  a  tube.  The  closure  of  the  blastopore  brings 
the  two  halves  of  the  plate  into  close  association  with  each  other,  making 
it  a  bilateral  structure.  With  bilaterality  comes  the  tendency  of  one 
end  of  the  animal  to  take  the  lead,  resulting  in  the  concentration  of 
nervous  and  sensory  structures  at  the  anterior  end,  which  first  comes 
in  contact  with  foreign  objects.  In  this  way  a  brain  has  been  special- 
ized apart  from  the  rest  of  the  nervous  system. 

With  the  appearance  of  metamerism  in  the  mesothelium  and  the 
development  of  muscles  from  the  myotomes  there  results  a  serial 
repetition  of  motor  nerves  going  to  these,  since  each  muscle  must  have 
its  own  ner\^e  supply,  while  sensory  nerves  are  the  result  of  the  sinking 
of  the  neural  plate  to  a  deeper  position,  as  the  sensory  organs  must  be 
largely  in  the  skin. 


138        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

The  close  association  of  sensory  and  motor  nerves  in  the  trunk  region  of  verte- 
brates is  not  yet  satisfactorily  explained.  The  fact  that  in  Amphioocus  the  two 
kinds  of  nerves  are  independent  of  each  other  throughout  their  course  shows  that 
the  vertebrate  condition  is  not  primitive. 

The  infolding  of  the  nervous  plate  has  been  described  (p.  11) 
and  with  that  stage  the  present  account  begins.  As  the  plate  is  broad- 
est in  front,  the  result  is  a  larger  anterior  portion  of  the  tube,  the  brain, 
while  the  rest  of  the  tube  gives  rise  to  the  spinal  cord.  Brain  and 
cord  constitute  the  central  nervous  system,  while  the  nerves  arising 
from  the  brain  and  cord  form  the  peripheral  nervous  system. 

CENTRAL  NERVOUS  SYSTEM. 

The  two  halves  of  the  neural  plate  are  separated  by  a  median  band 
of  non-nervous  tissue,  hence,  when  it  is  rolled  into  a  tube,  the  mid- 
ventral  line — the  floor  plate — is  thinner  and  differs  from  the  side  walls. 
With  the  closure  of  the  tube  (fig.  144,  A)  a  similar  roof  plate  appears, 


Fig.  144. — A,  diagram  of  early  spinal  cord;  B,  later,  showing  increase  in  size  and  con- 
sequent ventral  fissure,  c,  central  canal;  e,  ectoderm;/,  floor  plate;  g,  anlage  of  spinal 
ganglion;  nc,  neural  crest;  r,  roof  plate;  s,  sulcus  of  Monro;  v,  ventral  fissure. 

while  the  lumen  of  the  tube,  the  central  canal,  is  oval  in  section,  its 
side  walls,  consisting  of  embryonic  nervous  tissue,  being  thicker  than 
roof  or  floor. 

From  the  method  of  its  formation  it  will  be  seen  that  the  inner  surface  of  the 
neural  tube  is  homologous  with  the  outer  surface  of  the  general  epidermis  of  the 
body.  The  account  given  above  is  not  exact  for  cyclostomes,  teleosts  and  some 
ganoids,  where  the  neural  plate  forms  a  keel  extending  below  the  surface,  in 
which  a  central  canal  appears  later,  so  that  the  final  result  is  closely  similar  to 
the  typical  condition. 

The  Spinal  Cord. 

From  this  simple  tube  the  spinal  cord  of  the  adult  is  developed  by 
several  modifications.     The  cells  of  the  side  walls  proliferate  rapidly, 


NERVOUS   SYSTEM.  1 39 

while  those  of  roof  and  floor  plates  do  not.  As  a  result  the  sides  soon 
extend  downward  on  either  side  beyond  the  floor  plate,  thus  forming  a 
longitudinal  groove,  the  anterior  or  ventral  fissure  of  the  cord,  ex- 
tending its  whole  length  (fig.  144,  B).  The  roof  plate,  on  the  other 
hand,  is  at  first  carried  upward  by  the  growth,  thus  increasing  the  ver- 
tical diameter  of  the  central  canal.  Then  the  dorsal  portion  of  the 
tube  closes  up — the  exact  steps  are  uncertain — and  later  the  tissue 
along  the  line  of  closure  is  invaded  by  connective  tissue  and 
blood-vessels,  the  result  being  the  dorsal  or  posterior  fissure  of  the 
cord. 

Besides  the  increase  in  the  number  of  cells,  the  sides  of  the  cord  are 
modified  in  other  ways.  Those  cells  which  line  the  cavity — ^floor,  roof 
and  sides — retain  their  epithelial  character,  never  develop  nervous  struc- 
tures, and  are  known  as  the  ependjmia.  The  remaining  cells  become 
differentiated  in  two  directions.  Some  develop  processes  which  sur- 
round and  support  the  others,  these  forming  the  neuroglia  (*glia'), 
while  the  others  form  the  true  nervous  tissue — ganglion  or  nerve  cells. 
In  the  primitive  condition  the  primitive  nervT  cells  have  no  connexion 
with  distant  points  and  hence  cannot  function.  These  connexions  are 
established  by  protoplasmic  outgrowths  from  each  cell,  these  forming 
the  fibres  (dendrites  or  axons).  Some  of  these  extend  directly  out- 
ward from  the  cord  as  nenxs  (see  below) ,  but  others  run  for  a  greater 
or  less  distance  on  the  external  surface  of  the  cord,  and  since  these 
have  medullary  sheaths  (p.  20)  and  are  consequently  white,  these 
tracts  constitute  the  white  matter  of  the  cord,  in  contrast  to  the  gray 
matter  formed  by  the  cell  bodies  and  neuroglia. 

In  sections  of  the  adult  cord  the  gray  matter  has  something  of  the 
shape  of  the  letter  |— | ,  its  uprights  forming  the  anterior  and  posterior 
horns  or  cornua,  while  the  cross-bar  extends  above  and  below  the 
central  canal,  from  one  side  to  the  other.  Physiological  phenomena 
and  matters  of  nerve  origin  lead  to  the  recognition  of  a  lateral  cornu 
on  either  side,  in  the  lateral  prominence  of  gray  matter.  Since  both 
dorsal  and  ventral  cornua  approach  the  surface  of  the  cord  to  connect 
with  the  nerve  roots  described  below,  they  divide  the  white  matter  into 
three  tracts  on  either  side,  known  as  the  anterior,  lateral  and  pos- 
terior columns  of  the  cord,  each  subdivided  in  the  higher  vertebrates 
into  several  bundles.  As  this  white  matter  is  composed  of  nerve  fibres, 
it  follows  that  these  columns  are  the  tracts  by  which  nervous  impulses 
are  carried  to  and  from  the  brain,  the  anterior  columns  leading  from, 


I40        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  dorsal  to  the  brain  (ascending  and  descending  tracts),  while 
impulses  travel  in  both  directions  in  the  lateral  columns. 

In  fishes  the  cord  tapers  regularly  to  the  tip,  but  with  the  develop- 
ment of  legs  in  the  terrestrial  vertebrates  the  cord  is  considerably  en- 
larged where  the  nerves  to  the  limbs  are  given  off,  the  enlargements 
bearing  some  ratio  to  the  size  of  the  limb.  In  the  early  stages  the 
spinal  nerves  leave  the  cord  at  right  angles  to  its  axis.     With'  growth 


Fig.  145. — Diagram  of  spinal  cord  and  nerve  roots;  gray  matter  shaded.  A,  L,  P; 
anterior,  lateral  and  posterior  columns;  Ap,  anterior  pyramidal  tract;  B,  column  of  Burdach, 
cc,  central  canal;  ca,  cl,  cp,  anterior,  lateral  and  posterior  cornua;  Dr,  dorsal  root;  Fa,  Fp, 
anterior  and  posterior  fissures;  G,  column  of  Goll;  g,  ganglion  of  dorsal  root;  /c,  lateral 
cerebellar  tract;  Ip,  lateral  pyramidal  tract;  5«,  spinal  nerve. 

the  angle  changes  since  the  peripheral  parts  increase  more  in  length 
than  does  the  cord.  The  result  is  that  the  posterior  nerves  are  very 
oblique  and  in  the  hinder  part  of  the  spinal  canal  they  form  a  bimdle 
of  parallel  nerves,  the  cauda  equina.  Another  result  of  the  unequal 
growth  may  be  the  drawing  out  of  the  hinder  end  of  the  cord  into  a 
slender  non-nervous  thread,  the  filum  terminale. 

The  Brain. 

The  spinal  cord  throws  light  on  the  extremely  complex  brain. 
Here,  as  in  the  cord,  there  is  primitively  a  tubular  structure,  with  roof, 
floor  and  sides,  and  with  nerves  connected  with  it  w^hich  recall  those 
of  the  cord.  In  its  development,  as  stated  above,  the  brain  from  the  first 
is  larger  than  the  cord.  It  early  has  three  enlargements  separated  by 
two  constrictions,  the  third  enlargement  passing  gradually  into  the  cord. 
These  enlargements  are  called,  from  in  front  backward,  fore-brain, 


BRAIN. 


141 


mid-brain  and  hind-brain,  the  constriction  between  mid-  and  hind- 
brains  being  the  isthmus.  In  the  sides,  as  in  the  cord,  two  zones 
may  be  recognized,  dorsal  and  ventral,  separated  internally  by  a 
groove,  the  sulcus  of  Monro,  which  lies  at  about  the  middle  of  the 
tube.  At  the  extreme  anterior  end  a  small  region,  the  optic  recess, 
is"  wedged  in  between  the  two  zones  on  either  side,  the  end  of  the  tube 


Fig.  146. — Diagrams  of  (i)  primitive  brain.  (2)  an  intermediate  stage,  and'(3)  with 
the  definitive  parts.  (Compare  3  with  fig.  147).  AQ^  aqueduct;  AC^  anterior  commissure; 
C,  cerebral  region;  CB,  cerebeUimi;  CS,  corpus  striatum;  HC^  habenular  commissure; 
7,  infundibulum;  LT,  lamina  terminalis;  MO,  medulla  oblongata;  O,  olfactory  region; 
P,  epiphysial  region;  PC,  posterior  commissure;  RO,  optic  recess;  52",  subthalamica;  T, 
tegmentum;  TH,  thalamus.     Dorsal  zone  plain,  ventral  zone  dotted. 


just  above  the  recesses  being  the  lamina  terminalis.  The  most 
marked  modifications  in  converting  the  primitive  into  the  adult  brain 
take  place  in  the  dorsal  zone. 

In  the  fore-brain  the  anterior  part  of  the  dorsal  zone  on  either  side 
forms  an  outgrowth  which  rapidly  increases  in  size,  the  two  eventually 
forming  a  pair  of  hollow  vesicles,  the  cerebral  hemispheres  (telen- 
cephalon, prosencephalon)  which  extend  far  beyond  the  lamina 
terminalis.  In  the  wall  of  each  hemisphere  may  be  recognized  a  basal 
ganglionic  portion,  the  corpus  striatum,  while  the  rest  of  the  wall 
is  the  pallium  or  mantle.  An  olfactory  lobe  (rhinencephalon) 
grows  out  from  the  lower  anterior  part  of  each  hemisphere  to  meet 


142         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  olfactory  epithelium  (see  sense  organs),  and  into  this  a  portion  of 
the  cavity  (ventricle)  of  the  hemisphere  may  extend  (fig.  146). 

Considerable  differences  exist  in  the  olfactory  lobes.  In  some  cases  they  are 
directly  continuous  with  the  hemispheres,  but  they  may  be  prolonged,  each  having 
two  portions,  a  narrower  stalk,  the  tractus  olfactorius,  and  a  distal  enlargement, 
the  bulbus  olfactorius.  The  true  olfactory  nerve  takes  its  origin  from  the  end 
of  the  bulb  or  its  homologue  (for  details  see  cranial  nerves). 

The  ventral  zone  and  posterior  part  of  the  dorsal  zone  of  the  fore- 
brain,  after  the  differentiation  of  the  telencephalon,  forms  the  thala- 
mencephalon   ('twixt-brain,   diencephalon).     Its  sides,  the  optic 


Fig.  147. — Half  of  model  of  brain  of  embryo  pig,  15  mm.  long.  (Compare  with  fig. 
146,  3.)  c,  cerebrum;  cb,  cerebellum;  cs,  corpus  striatum;  i,  infundibulum;  is,  isthmus; 
fi,  interventricular  foramen;  m,  mesencephalon;  mo,  medulla  oblongata;  t,  thalamus. 

thalami,  remain  without  marked  modification,  but  its  floor  and  roof 
form  median  outgrowths,  corpus  pineale,  epiphysis,  etc.,  above,  in- 
fundibulum below — to  which  reference  will  be  made  again  later. 

In  the  mid-brain  there  is  little  modification  except  a  thickening  of 
the  walls  forming  a  pair  of  prominences,  the  optic  lobes,  or  corpora 
bigemina  (in  mammals  two  pairs  of  lobes,  corpora  quadrigemina) 
on  the  dorsal  surface.  The  mid-brain  of  the  adult  is  also  called  the 
mesencephalon  (fig.  146). 

In  the  hind-brain  the  great  modifications  occur  again  in  the  dorsal 
zone.  Its  dorsal  portion  extends  itself  upward  and  backward  as  a 
broad  lobe  which  tends  to  arch  over  the  rest  of  the  hind-brain.  This 
outgrowth  forms  the  cerebellum  or  metencephalon,  while  the 
remainder  of  the  hind-brain  constitutes  the  medulla  oblongata  or 
myelencephalon  (fig.  146). 

Thus  there  arise  in  the  adult  brain  five  regions — telencephalon, 
thalamencephalon,  mesencephalon,  metencephalon  and  myelencephalon 
— derived  from  the  primitive  three.     These  usually  retain  in  the  interior 


BRAIN. 


143 


the  cavity  of  the  primitive  three  (continuation  of  the  central  canal  of 
the  spinal  cord),  but  modified  in  different  ways.  The  cavity  in  the 
primitive  fore-brain  is  divided  with  the  outgrowth  of  the  hemispheres 
into  three  chambers  known  at  ventricles,  a  pair  of  cerebral  ventricles 
in  the  hemispheres  and  a  third  ventricle  in  the  thalamencephalon. 
The  paired  ventricles  are  connected  with  the  third  by  a  pair  of  narrower 
passages,  the  foramina  of  Monro  (for.  interventriculares).  In  the 
higher  vertebrates  the  cavity  of  the  mid-brain  becomes  reduced  to  a 
narrow  tube,  the  aqueduct  (or  iter),  but  in  the  lower  classes  (fig.  156) 
this  expands  dorsally  into  a  cavity,  the  epicoele,  in  the  upper  part  of  the 
optic  lobes.  The  aqueduct  terminates  behind  in  the  fourth  ventricle 
which  lies  in  the  hind-brain,  extending  forward  beneath  the  cerebellum 
and  gradually  diminishing  in  the  medulla  to  the  central  canal  of  the 
spinal  cord.  Sometimes  there  is  a  prolongation  of  the  fourth  ventricle 
into  the  cerebellum  (metacoele,  fig.  156). 


Fig.  148. — Median  section  of  brain  of  pig  15.5  mm.  long,  showing  flexures  of  the  brain 
C,  principal  flexure;  cs,  corpus  striaUmi;  CP,  chorioid  plexus  of  fourth  ventricle;  h,  hypo- 
physis; «,  infundibulum;  M,  mid-brain;  A^,  nuchal  flexure;  P,  pontal  flexure;  RO,  optic 
recess;  T,  'twixt-brain. 


So  far  the  brain  has  been  treated  as  if  it  were  a  continuation  of  the 
spinal  cord  in  a  straight  line.  In  reality,  by  unequal  growth  in  dorsal 
and  ventral  zones,  it  becomes  flexed  in  the  vertical  plane.  In  the  lower 
vertebrates,  these  flexures  never  attain  great  prominence  and  largely 
disappear  in  the  adult.  They  are  more  developed  in  the  higher  groups 
and  persist  throughout  life.  Most  constant  is  the  primary  flexure 
in  the  mid-brain,  by  which  the  derivatives  of  the  fore-brain  are  bent 
downward  at  a  right  angle  (or  more)  to  the  axis  of  the  rest.  Second 
to  appear  is  the  nuchal  flexure  in  the  hinder  part  of  the  medulla  ob- 


144 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


longata,  which  also  bends  in  the  same  direction.  The  pontal  flexure, 
beneath  the  cerebellum,  bends  in  the  opposite  direction  and  thus 
tends  to  counteract  the  other  two.  Nuchal  .and  pontal  flexures  are  at 
best  but  weakly  developed  in  the  ichthyopsida  and  all  are  practically 
obliterated  in  the  adult,  but  in  the  amniotes  they  are  increasingly 
developed  and  persist  through  life  (fig.  148). 

The  brain,  like  the  spinal  cord,  is  composed  of  nerve  cells  (gray 
matter)  and  fibres  (white  matter),  but  their  arrangement  is  exceedingly 
complicated  and  but  the  slightest  outline  of  their  distribution  can  be 
attempted  here,  in  connection  with  the  general  account  of  the  regions 
of  the  brain. 


Fig.  149. — Cross-section  of  medulla  of  Acanthias  embryo,  60  mm.  long,  showing  the 
greatly  broadened  roof  plate  and,  below,  a  bit  of  the  meninx  of  the  nervous  system,  c, 
cartilage  of  basal  plate;  e,  ependyma;  mp,  meninx  primitiva;  pc,  perichondrium  (endo- 
rhachis);  r,  roof  plate. 

The  myelencephalon  is  most  nearly  like  the  spinal  cord  of  any  part 
of  the  brain.  It  is  triangular  in  outline,  viewed  from  above,  and  is 
widest  anteriorly,  due  in  part  to  the  separation  of  the  side  walls  by  the 
great  development  of  the  roof  plate  over  the  fourth  ventricle.  Blood- 
vessels press  against  the  roof,  carrying  parts  of  it  before  them  into  the 
ventricle,  thus  forming  the  chorioid  plexus  of  the  fourth  ventricle,  a 
means  of  introducing  nourishment  into  the  brain.  (Usually  in  dis- 
sections this  roof  is  torn  away,  leaving  a  triangular  or  rhomboid  opening 
into  the  fourth  ventricle — fossa  rhomboidea).  The  floor  plate  in 
this  region  is  obliterated  by  the  development  of  numerous  nerve  centres 
— 'nuclei'  or  ganglia — in  the  walls,  some  closely  connected  with  the 


BRAIN. 


145 


fibre  tracts  soon  to  be  mentioned,  some  with  nerves  arising  from  this 
region. 

Most  noticeable  of  these  ganglia  are  the  olivary  bodies  (oliva)  near  the  roots  of 
the  hypoglossal  or  first  spinal  nerves;  the  nuclei  of  the  cuneate  and  slender  funiculi 
connected  with  the  posterior  columns  of  the  cord;  the  eminentia  medialis  in  the 
floor  of  the  fourth  ventricle,  connected  with  the  anterior  and  lateral  columns;  and 
the  tuber  acusticum,  an  enlargement  connected  with  the  eighth  nerve;  its  anterior 
end  in  the  ichthyopsida  is  specialized  as  the  lobe  of  the  lateral  line  system 

The  cerebellum  is  developed  from  the  dorsal  zones  and  the  roof 
plate,  the  latter  invaded  by  nerve  cells  from  the  sides.  In  front  it  dips 
deeply  into  the  fourth  ventricle,  its  anterior  portion  being  vertical  and 
together  with  part  of  the  roof  of  the  isthmus,  forming  the  valve  of 


Fig.  150. — Diagrammatic  longitudinal  section  of  brain,  ac,  anterior  commissure  in 
lamina  terminalis;  aq,  aqueduct;  c,  cerebrum;  ch,  cerebellum;  cp,  chorioid  plexus;  cs,  corpus 
striatum;  cv,  cerebellar  ventricle;  A,  hypophysis;  he,  habenular  commissure;  «/>,  inferior 
chorioid  plexus;  m,  mesencephalon;  ml,  myelencephalon;  p,  pinealis;  pa,  paraphysis;  /»c, 
posterior  commissure;  pe,  parietal  eye;  v,  valve  of  Vieussens;  vt,  velum  transversum  with 
aberrant  commissure. 

Vieussens  (velum  medullare  anterius,  fig.  150).  In  the  ichthyopsida 
and  lower  reptiles  there  is  no  special  differentiation  of  parts  in  the  cere- 
bellum, but  in  the  higher  reptiles  and  in  the  birds  a  central  portion,  the 
vermis,  and  a  pair  of  lateral  lobes,  the  flocculi  (fig.  161)  occur.  In  the 
mammals  the  cerebellum  is  still  farther  enlarged,  chiefly  by  the  develop- 
ment of  large  cerebellar  hemispheres  between  vermis  and  flocculi,  the 
latter  being  forced  by  them  to  the  lower  side  of  the  cerebellum.  In  the 
walls  of  each  hemisphere,  besides  others,  there  is  a  large  nerve  centre, 
the  nucleus  dentatus,  connected  with  the  posterior  peduncle  of  the 
cerebellum  to  be  mentioned  shortly,  and  with  the  fibres  which  go 
farther  forward  in  the  brain. 

The  mesencephalon  is  relatively  largest  in  the  lower  vertebrates, 
less  conspicuous  and  tending  to  be  covered  by  cerebrum  and  cerebellum 
in  the  higher  groups.  On  its  dorsal  surface  are  the  two  optic  lobes 
(transversely  divided  in  the  mammals)  each  connected  with  an  optic 


146 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


tract  leading  to  the  eye  of  the  opposite  side.  In  the  lower  groups  the 
lobes  contain  an  epicoele  (p.  143),  but  in  the  higher  they  are  solid,  the 
cavity  being  reduced  to  the  aqueduct.  The  floor  of  the  mid-brain  is 
formed  of  large  fibre  tracts  (see  below),  the  floor  plate  having  been 
invaded  by  their  fibres. 

In  the  thalamencephalon  ('twixt-brain)  the  lateral  walls  are  thick- 
ened, the  dorsal  zones  developing  a  nerve  centre,  the  optic  thalamus, 


Fig.  151. — ^Parietal,  eye  of  Anguis  fragilis,  after  Nowikoff.  ct,  connective  tissue  cells 
around  nerve;  gc,  ganglion  cells;  /,  lens;  w,  nerve  fibres;  pn,  parietal  nerve;  pc,  pigment  cells; 
r,  retinal  cells;  vh,  vitreous  body. 


on  either  side.  These  are  ganglionic  and  are  closely  related  to  the 
corpora  striata.  Frequently  the  thalami  of  the  two  sides  touch  or 
even  unite  above,  forming  the  so-called  soft  commissure  (commis- 
sura  mollis,  fig.  152) — really  not  commissural  in  character.  Still 
more  dorsal  is  a  small  habenular  ganglion  on  either  side,  in  front 
of  the  pinealis  to  be  described  in  a  moment. 

Under  the  head  of  epiphysial  structures  are  several  parts  devel- 
oped in  the  roof  plate  of  the  primitive  fore-brain.  At  the  junction  of 
cerebral  hemispheres  and  twixt-brain  (fig.  150)  there  is  an  internal  epi- 
thelial fold,  the  veltmi  transversum,  depending  from  the  cerebral 
roof.  In  front  of  this  an  outgrowth,  the  paraphysis,  arises  on  the  top 
of  the  brain  in  nearly  all  vertebrates.  It  is  non-nervous  and  apparently 
is  an  extra-ventricular  chorioid  plexus  with  secretory  functions.  The 
other  epiphysial  structures  belong  to  the  'twixt-brain  and  consist  of  a 
parietal  organ  and  a  pinealis.     Both  arise  from  the  roof  between  the 


BRAIN.  147 

habenular  ganglion  and  the  posterior  commissure,  at  the  boundary 
between  'twixt-  and  mid-brains,  sometimes  as  two  distinct  structures, 
sometimes  as  the  result  of  division  of  a  single  outgrowth  of  the  roof. 
The  anterior  of  these  is  the  parietal  organ  or  eye;  the  other  the 
pinealis  or  epiphysis  proper.  The  two  vary  in  development  in  dif- 
ferent vertebrates,  the  parietal  eye  being  well-marked  only  in  cyclos- 
tomes,  Amia,  teleosts  and  most  lizards  (fig.  151),  while  the  pinealis 
is  almost  invariably  present. 

In  its  fullest  development  in  lizards  and  Sphenodon  the  parietal 
organ  extends  as  a  slender  stalk,  hollow  at  first,  through  the  parietal 
foramen  of  the  skull,  expanding  beneath  the  skin  to  a  vesicle,  above 
which  the  integument  is  usually  thin  and  transparent,  forming  a  physi- 
ological cornea.  The  distal  wall  of  the  vesicle  is  thickened  in  the 
middle,  forming  a  lens,  while  the  cells  of  the  proximal  side  elongate, 
each  becoming  differentiated  into  a  distal,  rod-like  end  and  a  proximal 
portion  w^hich  contains  the  nucleus  and  is  connected  with  a  nerve  fibre. 
Pigment  is  deposited  between  these  cells  so  that  the  whole  forms  a 
retina.  An  important  point,  to  be  better  appreciated  after  the  con- 
sideration of  the  paired  eyes,  is  the  fact  that  these  parietal  eyes  are 
like  those  of  most  invertebrates  in  having  no  inversion  of  the  retina. 
How  far  these  eyes  are  actually  fimctional  is  not  settled.  Even  in 
Sphenodon,  where  it  is  best  developed,  experiments  have  resulted  in  no 
decided  reactions. 

In  other  vertebrates  the  parietal  organ  does  not  pass  outside  the 
skull,  and  even  may  not  appear  transitorily  in  development.  The 
pinealis  to  some  extent  may  take  its  place  and  often  shows,  as  in  certain 
lizards,  traces  of  a  visual  structure.  In  the  anura  its  tip  approaches 
the  skin  and  later  is  cut  off  from  the  brain  by  the  development  of  the 
skull,  forming  the  so-called  frontal  organ,  visible  from  the  exterior. 
Pineal  and  parietal  organs  differ  in  their  nervx  supply,  the  parietal  being 
connected  with  the  superior  commissure  of  the  ^twixt-brain,  the 
pinealis  and  its  derivatives  with  the  posterior  commissure.  In  the 
higher  vertebrates  the  epiphysial  structures  are  completely  covered  by 
the  backward  growth  of  the  cerebrum.  The  large  parietal  foramina 
in  many  extinct  reptiles  would  seem  to  indicate  that  they  had  well 
developed  parietal  or  pineal  organs.  The  roof  of  the  brain  in  this 
region,  behind  the  lamina  terminalis,  also  gives  rise  to  a  chorioid  plexus 
like  that  of  the  fourth  ventricle,  a  part  of  which  invades  the  third 
ventricle  and  another  portion,  the  inferior  plexus,  sends  branches 


148        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

through  the  foramina  interventriculares  into  the  ventricles  of  the  hemi- 
spheres, thus  providing  for  a  blood  supply  on  the  interior  of  these 
structures  (fig.  150), 

The  floor  of  the  diencephalon  remains  thinner  behind  the  optic 
recess,  a  portion  of  it  becoming  funnel-shaped  and  pushing  out  from  the 
ventral  surface  toward  the  roof  of  the  mouth.  This  is  the  infundib- 
ulum  which  meets  an  ectodermal  diverticulum,  the  hypophysis. 
This  arises,  in  the  cyclostomes  from  the  ectoderm  between  the  nostril 
and  the  mouth;  in  other  vertebrates  from  the  roof  of  the  oral  cavity. 
It  retains  its  connection  with  the  parent  epithelium  for  a  time,  the  point 
of  ingrowth  being  known  as  Rathke's  pocket.  Later  the  stalk  dis- 
appears and  the  infundibulum  and  hypophysis,  closely  associated,  lie 
just  beneath  the  brain  in  the  sella  turcica  on  the  floor  of  the  skull 
(p.  61).  In  the  hypophysis  (pituitary  body)  two  parts  are  distin- 
guished, rich  in  blood-  and  lymph-vessels  and  forming  a  gland  of  internal 
secretion  whose  action  is  connected  with  the  fat-storing  powers  of  the 
animal.  The  infundibulum  may  be  a  simple  pit,  as  in  most  vertebrates, 
or  its  lateral  walls  may  become  enlarged  and  folded,  blood-vessels 
lying  in  the  folds,  and  the  whole  forming  the  so-called  saccus  vascu- 
losus.  The  paired  eyes  are  also  connected  with  the  'twixt-brain,  both 
in  origin  and  in  the  adult;  they  are  described  with  the  other  sense 
organs. 

The  cerebrum  (telencephalon)  consists  of  a  pair  of  hemispheres, 
separated  in  front  by  an  inter  cerebral  fissure,  slight  in  fishes,  well 
marked  in  other  vertebrates.  Each  hemisphere  typically  contains  a 
ventricle,  the  walls  of  which  are  formed  by  the  corpus  striatum  below 
and  elsewhere  by  a  thinner  portion,  the  pallium  or  mantle.  To  the 
roof  belong  the  paraphysis  and  the  inferior  chorioid  plexus,  already  men- 
tioned. In  some  vertebrates,  like  the  teleosts,  the  whole  of  the  pallium 
remains  thin  and  epithelial  throughout  life;  elsewhere  it  is  invaded  to  a 
greater  or  less  extent  by  nervous  matter.  In  the  amphibia  and  reptiles, 
where  the  olfactory  lobes  are  merged  in  the  hemispheres,  the  medial 
wall  of  each  hemisphere  as  far  back  as  the  interventricular  foramen  is 
dalled  the  septum,  while  the  part  above  the  foramen,  together  with  the 
posterior  dorsal  and  lateral  walls,  is  to  be  regarded  as  homologous  with 
a  region,  long  recognized  only  in  mammals,  the  hippocampus,  connected 
with  the  olfactory  sense.  In  the  mammals  a  new  element,  the  neopal- 
litmi,  appears  in  the  cerebrum.  In  the  lower  groups  it  is  on  the  outer 
wall,  behind  the  olfactory  tract,  and,  increasing  in  extent  in  the  higher 


BRAIN. 


149 


groups,  forces  the  hippocampus  to  the  medial  side  of  the  hemisphere. 
Other  modifications  are  better  imderstood  after  a  consideration  of 
the  commissures  of  the  brain. 

The  amount  of  gray  matter  in  the  pallium  is  evidently  correlated  with  the  mental 
powers  of  the  animal,  being  greatest  in  the  mammals.  Here  the  nerve  cells  form 
a  layer  (cortex)  on  the  surface  of  the  neopallium.  Increase  in  the  number  of 
these  cells  can  be  accommodated  to  some  extent  by  increase  in  the  size  of  the 
cerebrum,  but  the  extent  of  this  increase  is  limited,  and  in  the  higher  mammals  the 
amount  of  surface  is  increased  by  folding,  so  that  the  cerebrum  is  marked  extern- 
ally by  numerous  fissures  or  sulci  separating  convolutions  or  gyri,  as  will  be 
mentioned  in  the  paragraphs  on  the  mammalian  brain. 

In  order  that  the  two  sides  of  the  body  may  work  in  harmony  it  is 
necessary  that  the  right  and  left  side  of  the  central  nervous  system  be 


Fig.  152. — Medial  plane  of  brain  of  Ornithorhynchus,  after  G.  Elliot  Smith,  ac,  anterior 
commissure;  bo,  bulbus  olfactorius;  cm,  commissura  mollis;  d,  cerebellum;  e,  epiphysis; 
fd,  fasciculus  dentatus;/*,  interventricular  foramen;  h,  hypophysis;  he,  habenular  commis- 
sure;  Ip,  lobus  pyriformis;  mc,  corpus  mamillare;  md,  medulla  oblongata;  n,  nodulus;  ol, 
olfactory  lobes;  oi,  olfactory  tubercle;  pal,  pallium;  pc,  posterior  commissure;  ic,  tuber 
cinereum;  v,  velum  medullare;  vmo,  motor  root  of  fifth  nerve;  vm,  maxillary  of  fifth. 

connected.  This  is  accomplished  in  the  spinal  cord  by  nerve  fibres 
which  pass  above  and  below  the  central  canal  from  one  side  to  the  other. 
In  the  brain  these  commissures  are  more  localized.  Then  there  are 
longitudinal  fibre  tracts  in  the  brain,  some  of  which  are  continuous  with 
the  columns  of  the  cord  already  mentioned.  Only  a  few  of  these  connex- 
ions, which  are  more  numerous  in  the  higher  than  in  the  lower  verte- 
brates, can  be  mentioned  here. 

Most  constant  and  important  of  the  commissures  are  the  following: 


150        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

In  the  lamina  terminalis,  a  little  below  the  interventricular  foramen, 
an  anterior  commissure,  connecting  the  two  hemispheres;  a  poster- 
ior commissure  in  the  roof  at  the  junction  of  di-  and  mesencephalon; 
and  a  superior  or  habenular  commissure  associated  with  the  habenular 
ganglia  and  lying  between  the  epiphysial  structures  and  the  velum 
transversum.  In  the  amphibia,  with  the  differentiation  of  the  hip- 
pocampal  region,  a  dorsal  or  hippocampal  commissure  appears  in 
the  lamina  terminalis,  just  dorsal  to  the  anterior  commissure,  connecting 
the  hippocampi  of  the  two  sides.  This  persists,  with  slight  modifica- 
tions, through  the  sauropsida  and  monotremes,  but  in  the  higher 
mammals  it  is  subdivided  into  the  hippocampal  commissure  proper  and 
a  more  anterior  portion,  the  corpus  callosum.  This  corpus  callosum 
is  only  in  part  the  result  of  the  division,  but  is  more  largely  formed  by 
new  fibres,  anterior  to  the  hippocampal  portion,  connecting  the  neopal- 
lium of  the  two  sides.  The  result  is  a  broad  band  (the  largest  com- 
missure in  the  brain  of  man)  which  invades  the  intercerebral  fissure 
from  behind.  In  the  lower  vertebrates  a  few  fibres  pass  downward  from 
either  side  of  the  cerebellum  beneath  the  fibre  tracts  of  the  medullary 
region  and  so  to  the  other  side  of  the  cerebellum.  In  the  mammals 
these  are  greatly  increased  in  number,  forming  a  marked  projection  on 
the  lower  surface,  the  pons  (Varolii),  the  prominence  of  which  is  in- 
creased by  the  great  development  of  '  nuclei '  in  the  medullary  floor. 

The  longitudinal  tracts  are  more  numerous  and  more  complex. 
As  will  be  recalled,  there  are  dorsal,  lateral  and  ventral  columns  in 
the  spinal  cord.  These  extend  into  the  medulla  oblongata  and  there 
pursue  different  courses. 

Some  of  the  fibres  of  the  dorsal  columns  end  in  connection  with  the  nuclei  of 
the  medulla  (p.  144),  while  others  unite  with  fibres  from  the  lateral  column  and  with 
some  from  the  oliva  to  form  an  enlargement,  the  corpus  restifonne,  and  then 
bend  upward  (posterior  peduncle)  to  enter  the  cerebellum.  Other  fibres  from 
the  lateral  column,  together  with  some  from  the  dentate  nucleus,  enter  the  cere- 
bellum farther  in  front  as  the  anterior  pedimcle,  those  from  the  dentate  nucleus 
pass  forward  to  the  roof  of  the  mid-brain,  some  terminating  in  the  optic  lobes, 
others  continuing  to  the  cerebrum.  In  this  forward  course,  after  leaving  the  cere- 
bellum, the  fibres  cross  (decussate),  those  from  the  right  side  passing  to  the  left 
side  of  the  brain  farther  forward  and  vice  versa.  In  the  dorsal  region  of  the  medulla 
there  is  a  short  tractus  solitarius  (fasciculus  communis)  derived  from  fibres 
from  the  seventh  to  tenth  nerves  and  extending  no  farther  forward  than  the  seventh. 

In  the  higher  vertebrates  there  are  the  crossed  and  the  direct  pyramidal  tracts 
on  the  ventral  side  of  the  medulla,  the  direct  being  continuations  of  part  of  the  ven- 
tral columns,  the  crossed  of  the  deeper  lateral  columns.     In  the  medulla  these  en- 


BRAIN. 


151 


large  and  become  somewhat  pyramidal,  the  enlargement  being  due  in  part  to  the 
decussation  of  the  crossed  tracts.  The  tracts  pass  forward  from  the  decussation 
and  in  the  mid-brain  region  they  diverge  to  pass  the  hypophysial  structures  farther 
in  front,  the  diverging  portions  being  called  the  crura  cerebri.  The  fibres  of  the 
crura  enter  the  corpora  striata  and  in  the  mammals,  the  cerebral  cortex. 

The  direct  pyramidal  tracts  have  no  decussation  in  the  medullary  region,  but 
pass  to  the  hemisphere  of  the  same  side;  the  fibres,  however,  do  cross  in  the  spinal 
cord.  Recently  attention  has  been  called  to  Reissner's  fibres  which  occur  in  all 
vertebrates,  but  are  relatively  largest  in  fishes.  They  arise  from  the  roof  of  the 
mid- brain,  descend  to  the  aqueduct  and  pass  through  the  fourth  ventricle  and  into 
the  central  canal  to  terminate  at  various  points  in  the  region  of  the  spinal  nerves. 
It  has  been  suggested  that  they  afford  a  short  cut  for  visual  reflexes.  Another 
supposition  is  that  they  regulate  the  flexion  of  the  body. 

Of  the  numerous  longitudinal  tracts  in  the  anterior  part  of  the  brain 
the  fornix  must  be  mentioned.  It  appears  first  in  the  amphibia  and 
is  well  developed  in  the  mammals.  Its  fibres  are  connected  in  front 
with  the  hippocampus,  pass  downward  through  the  lamina  terminalis 
to  the  floor  of  the  third  ventricle,  where  they  produce  a  marked  swelling 
(corpus  albicans)  on  either  side  of  the  ventral  surface  of  the  dien- 
cephalon.  They  ascend  from  this  point  to  the  optic  thalami.  The 
passage  of  the  tracts  of  the  fornix  through  the  lamina  terminalis  and 
the  forward  growth  of  the  corpus  callosum  stretch  the  lamina  into  a 
thin  triangular  area,  the  septum  pellucidimi,  and  at  the  same  time  the 
callosum  causes  the  lamina  to  split,  the  enclosed  cavity  being  called 
the  *  fifth  ventricle'  though  it  has  no  relation,  physical  or  mor- 
phological, with  the  true  ventricles  of  the  brain. 

ENVELOPES    (MENINGES)    OF    THE    CENTRAL    NERVOUS 

SYSTEM. 

Both  brain  and  spinal  cord  are  surrounded  by  envelopes  (meninges) 
of  connective  tissue  which  support  and  protect  them,  and  also,  by 
carrying  blood-vessels,  provide  for  their  nourishment.  These  meninges 
become  more  complicated  with  ascent  in  the  vertebrate  series.  The 
canal  of  the  vertebral  column  and  the  cavity  of  the  skull  are  lined  with 
a  layer  of  connective  tissue,  the  endorhachis,  which  is  really  the  perios- 
teum or  perichondrium  of  the  skeletal  parts  and  hence  not  a  true  meninx. 
In  the  fishes  (fig.  149)  there  is  a  single  envelope,  the  meninx  primi- 
tiva,  which  bears  the  blood-vessels  and  lies  close  upon  the  spinal  cord. 
Between  it  and  the  endorhachis  is  a  perimeningeal  space,  somewhat 
broken  by  strands  of  tissue  passing  from  meninx  to  endorhachis,  and 


152        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

filled,  like  all  meningeal  spaces,  with  an  albumen-containing  cerebro- 
spinal fluid. 

From  the  urodeles  upward  there  is  an  increasing  division  of  the 
meninx  primitiva  into  two  layers,  a  pia  mater  bearing  the  blood- 
vessels and  lying  close  to  the  cord,  and  a  dura  spinalis,  separated  from 
the  pia  by  a  subdural  space,  the  perimeningeal  space  now  being  known 
as  the  peridural.  In  the  mammals  the  pia  becomes  invaded  by  cavities 
separating  a  delicate  arachnoid  membrane  from  its  outer  surface,  so 
that  there  is  another  space,  the  subarachnoid,  in  these  forms. 

There  may  be  slight  differences  in  the  region  of  the  brain  in  the 
higher  groups  where  the  dura  presses  against  and  finally  unites  with  the 
endorhachis,  forming  the  dura  mater  of  human  anatomy,  thus  obliterat- 
ing the  subdural  space.  In  the  mammals  and  to 
fa  less  extent  in  birds  the  dura  mater  forms  two 
strong  folds.  One  of  these  is  longitudinal  and 
presses  in  between  the  two  cerebral  hemispheres  as 
a  firm  membrane,  the  falx  cerebri.  The  other 
fold,  the  tentorium,  is  transverse,  and  is  inserted 
between  cerebrum  and  cerebellum.  It  is  occasion- 
ally ossified  and  united  to  the  skull. 
The  Brain  in  the  Separate  Classes. 
CYCLOSTOMES.— The  brain  is  very  different  in  the 
two  classes  of  cyclostomes.  All  parts  lie  in  the  same  hori- 
zontal plane,  the  flexures  having  disappeared,  and  the 
whole  presents  a  primitive,  almost  embryonic  appearance. 
In  the  lampreys  the  somewhat  slender  brain  is  elongate 
and  its  roof  is  largely  epithelial,  this  extending  to  the  mid- 
brain, of  which  only  the  hinder  part  is  nervous  in  the  middle 
line.  The  small  cerebral  hemispheres  are  largely  com- 
FiG  it:^— Brain  of  P°^^^  ^^  ^^^  corpora  striata  and  the  dorsal  part  of  the 
Bdellostoma  (Princeton,  pallium  is  purely  epithelial,  the  ventricles  being  well  de- 
2204).  o,  skeleton  of  yd  oped  and  extending  into  the  olfactory  lobes.  The 
brain  behind  tiiis- V-X  ^P^^^  lobes  and  the  medulla  are  relatively  broad,  but 
nerves.  the  cerebellum  is  reduced  to  an  inconspicuous  fold  in  front 

of  the  fossa  rhomboidea. 
Authors  do  not  agree  regarding  the  interpretation  of  some  parts  of  the  myxinoid 
brain.  The  whole  is  much  broader  and  shorter  than  in  the  other  class  and  is 
marked  dorsally  by  a  groove  running  the  whole  length.  According  to  Retzius,  the 
'twixt-brain  of  Myxine  is  invisible  from  above  and  the  cerebellum  is  large,  com- 
pletely covering  the  fossa  rhomboidea.     The  cavities  are  greatly  reduced,  the 


BRAIN. 


153 


aqueduct  ending  blindly  in  the  mid-brain,  in  front  of  which  is  only  the  third  ven- 
tricle, completely  cut  ofif  from  the  rest.  The  brain  of  Bdellostoma  (fig.  153) 
differs  from  this  in  several  respects. 

ELASMOBRANCHS  (figs.  154,  167)  usu- 
ally have  the  brain  somewhat  compact,  but  in 
a  few  it  is  long  and  slender.  The  more  strik- 
ing features  are  the  slight  development  of  the 
intercerebral  fissure,  the  large  hemispheres  be- 
ing lateral  expansions  just  in  front  of  the  dien- 
cephalon.  The  optic  lobes  are  large  and  the 
large  cerebellum  overlaps  both  lobes  and  the 
fossa  rhomboidea.  The  olfactory  lobes  arise 
from  the  antero-lateral  angle  of  each  hemi- 
sphere; their  length  varies  between  wide  limits. 
The  epithelial  roof  of  the  'twixt-brain  is  wide 
and  bears  a  pinealis  which  often  reaches  the  roof 
of  the  skull,  but  the  parietal  organ  is  lacking. 
The  hypophysis  and  infundibulum  are  pro- 
vided with  large  inferior  lobes  and  a  well  devel- 
oped saccus  vasculosus.  The  cerebellum  has  a 
longitudinal  groove  and  usually  one  or  more 
transverse  grooves,  dividing  the  upper  surface 
into  paired  lobes.  The  medulla  dififers  in  the 
sharks  and  the  skates,  being  very  short  in  the 
latter,  much  longer  in  the  former.  In  both  the 
corpora  restiformia  are  large  folds  on  either  ^^x, 
side  of  the  cerebellum,  in  front  of  and  lateral 
to  the  fossa  rhomboidea. 

In  most  elasmobranchs-the  ventricular  sys- 
tem is  well  developed,  but  in  some  the  paired 
and  third  ventricles  are  not  well  separated, 
while  in  the  Myliobatidae  there  is  no  cavity  in 
the  cerebrum.  There  is  a  large  epicoele  ex- 
tending upward  from  the  aqueduct  into  the 
optic  lobes  and  a  similar  cavity  usually  enters  //, 
the  cerebellum. 

TELEOSTOMES.—There  is  a  wide  range 
of  form  in  the  brain  of  ganoids  and  teleosts.  It 
is  usually  small  in  proportion  to  the  size  of  the 
animal  and  is  noticeable  for  the  small  size  of 
the  telencephalon  and  the  usually  non-nervous 
character  of  the  pallium,  which  in  the  teleosts  is 
purely  epithelial.     Consequently  the  cerebrum 

consists  largely  of  the  corpora  striata  and  the  intercerebral  fissure  is  slightly  de- 
veloped. The  paired  ventricles  are  small,  but  they  extend  into  the  olfactory  lobes. 
The  'twixt-brain,  at  a  lower  level  than  the  rest,  has  a  large  infundibulum,  saccus 


Fig.  154. — Brain  of  Heptanchus, 
after  Gegenbaur.  ho,  bulbus  olfac- 
torius;  c,  cerebrum;  ch,  cerebellum; 
em,  eminentla  teretes;  /,  inftmdibu- 
lum;  m,  mesencephalon;  00,  olfactory 
organ;  ot,  olfactory  tract;  my,  myelen- 
cephalon;  /,  'twixt-brain;  II-X,  cra- 
nial nerves. 


154 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


Fig.  155. — Dorsal  and  side  views  of  brain  of  buffalo  fish  (Carpiodes  tumidus)  after 
Herrick.  c,  cerebrum;  cl,  cerebellum;  cs,  corpus  striatum;  h,  hypophysis;  i,  infundibulum; 
ol,  olfactory  lobes;  p,  pallium;  vl,  vagus  lobes;  II~X,  nerves. 


cl 


M 


V     Jfn,      D 


bo   r 


df  sv  I'i  h         Ch    <^i 


ca.  CS 


Fig.  156. — Sagittal  section  of  brain  of  trout,  after  Rabl-Ruckhard.  aq,  aqueduct;  bo, 
bulbus  olfactorius;  ca,  ch,  ci,  cp,  anterior,  horizontal,  inferior  and  posterior  commissures; 
CO,  central  canal;  cl,  cerebellum;  h,  hypophysis;  i,  infundibulum;  oc,  optic  chiasma;  p, 
pallium;  pi,  pinealis;  sv,  saccus  vasculosus;  tl,  torus  longitudinalis;  to,  tectum  of  optic 
lobes;  v^,  v*  ventricles;  vc,  valvula  cerebelli. 


BRAIN.  155 

vasculosus  and  inferior  lobes.  On  its  roof  is  a  large  pinealis  which  reaches  the  skull  in 
a  few  ganoids.  The  parietal  organ  appears  in  the  embryo  and  soon  degenerates;  the 
paraphysis  is  usually  well  developed.  The  optic  lobes  are  large  and  are  usually 
divided  into  two  hemispheres  by  a  median  groove,  but  this  occasionally  is  scarcely 
noticeable.  The  cerebellum  is  large,  much  larger  than  appears  from  the  surface, 
since  a  considerable  part,  the  valvula,  projects  into  the  ventricle  of  the  mid-brain. 
In  the  cerebellar  region  there  is  sometimes  an  enormous  development  of  the  lobes 
of  the  vagus  (fig.  155). 

The  brain  of  Polypterus  differs  from  that  of  other  ganoids  in  several  respects. 
There  is  no  dififerentiation  of  cerebral  hemispheres;  the  optic  lobes  and  the  cerebel- 
lum are  moderate,  the  latter  being  thin  in  the  median  line  and  the  valvula  smaller. 
The  medulla  oblongata  has  thin  walls  and  the  ventricle  is  large.  The  brain  has  a 
primitive  appearance,  but  it  shows  little  resemblance  to  those  of  the  amphibia  or 
of  the  dipnoi. 

DIPNOI. — The  brains  of  Lepidosiren  and  Protopierus  differ  considerably  from 
that  of  Ceratodus.     In  all  the  cerebrum  is  larger  than  the  optic  lobes  and  the 


Fig.  157. — Brain  of  Protopterus,  after  Burckhardt.  cb,  cerebellum;  e,  epiphysial 
structiures;  h,  hypophysis;  i,  infundibulum;  m,  mid  brain;  5e,  sacois  endolymphaticus;  sp, 
spinal  nerves;  t,  cerebrum;  1-12,  cranial  nerves. 

olfactory  bulb  is  separated  from  the  cerebrum  by  a  long  olfactory  tract.  In  Cer- 
atodus the  hemispheres  are  united  above  by  a  part  of  the  chorioid  plexus,  while 
internally  they  are  separated  from  the  diencephalon  by  a  well  marked  velum.  The 
pinealis  is  long  and  rests  upon  a  large  'zirbelpolster'  developed  as  an  outgrowth 
of  the  roof  of  the  third  ventricle  in  front  of  the  superior  commissure.  The  optic 
lobes  are  separated  into  two  hemispheres^  while  the  cerebellum  is  scarcely  more  than 
a  transverse  plate  and  is,  together  with  the  fossa  rhomboidea,  covered  with  a  com- 
plicated chorioid  plexus.  In  Protopterus  (fig.  157)  the  elongate  hemispheres  are 
parallel,  the  pinealis  and  its  'polster'  are  smaller  and  the  mid-brain  has  but  a 
single  rounded  lobe. 

AMPHIBIA. — ^The  parts  of  the  amphibian  brain  are  more  distinct  from  each 
other  than  is  usual  in  vertebrates,  and,  except  in  the  gymnophiones,  the  flexures 
have  largely  disappeared  in  the  adult.  There  is  a  deep  intercerebral  fissure 
between  the  hemispheres,  but  in  the  anura  the  two  halves  of  the  cerebrum  are 
connected  by  a  transverse  band  just  behind  the  olfactory  lobes.  The  telencephalon 
is  relatively  larger  than  in  fishes,  the  increase  being  due  to  the  invasion  of  the  pal- 
lium by  nervous  matter,  while  the  corpora  striata  are  relatively  smaller  than  in  other 
ichthyopsida.  In  the  pallium  the  inner  part  is  largely  composed  of  nerve  cells, 
the  outer  layer  consisting  of  nerve  fibres. 

The  diencephalon,  broad  in  the  anura,  narrower  in  the  urodeles  and  caecilians, 


156 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


is  visible  from  above.  The  infundibulum  and  hypophysis  are  well  developed  but 
the  saccus  vasculosus  and  inferior  lobes  are  smaller  than  in  fishes.  In  the  gymno- 
phiones,  owing  to  the  pontal  flexure  the  hypophysis  is  carried  back  beneath  the 
medulla  oblongata.     Both  paraphysis  and  pinealis  are  present,  the  latter  not  reach- 


Fig.  158. — Dorsal  and  ventral  views  and  sagittal  section  of  brain  of  DesmogncUhus,  after 
Fish,  a,  anterior  commissure  and  rudimentary  corpus  callosum;  c,  cerebrum;  d,  cere- 
bellum; e,  epiphysis;  h,  hypophysis;  z,  infundibulum;  oc,  optic  chiasma;  ol,  optic  lobes;  p, 
paraphysis;  pc,  posterior  commissure;  cp,  chorioidplexuses ;  sc,  superior  commissure;  I-X, 
nerves. 

ing  the  cranial  roof  except  in  the  anura,  the  conditions  in  this  group  having  already 
been  mentioned  (p.  147).  The  cerebellum  is  very  small,  a  mere  transverse  fold 
on  the  anterior  border  of  the  fossa  rhomboidea.  The  gymnophione  brain  is  notice- 
able for  the  pontal  flexure  already  alluded  to,  which  carries  the  hemispheres  so  far 


BRAIN. 


157 


back  that  they  almost  touch  the  sides  of  the  medulla,  and  for  the  double  roots  of  the 
olfactory  nerves. 

REPTILES. — There  is  considerable  range  in  the  brain  of  the  reptiles,  all  show- 
ing an  advance  over  the  amphibians  in  having  the  cerebrum  larger  than  the  optic 
lobes;  in  having,  in  the  pallium,  besides  the  basal  layer  of  gray  matter,  a  distinct 
cortical  layer  of  nerve  cells;  the  well  developed  hippocampus;  while  the  olfactory 
lobes  may  either  be  sessile  upon  the  hemispheres  or  differentiated  into  tracts  and 
bulbs. 


Fig.  159.  Fig.  160. 

Fig.  159. — Brain  of  Iguana  tuherctdata  (Princeton,  2293).     Compare  fig.  172. 

Fig.  160. — Side  and  dorsal  views  of  brain  of  young  aL&gator,  after  Herrick.  c,  cere- 
brum; cl,  cerebellum;  e,  epiphysial  structures;  h,  hypophysis;  »,  infundibulum;  ol,  olfactory 
lobes;  II-XII,  cranial  nerves. 


The  greater  size  of  the  cerebrum  and  the  large  optic  lobes  result  in  covering  the 
diencephalon  so  that  it  is  scarcely  visible  from  above  (figs.  159, 160).  Infundibulum 
and  hypophysis  are  well  developed,  but  the  sacci  vasculosi  are  rudimentary  and  the 
inferior  lobes  are  inconspicuous.  The  epiphysial  structures  reach  their  highest 
development  in  this  group.  In  most  species  the  parietal  organ  is  rudimentary,  but 
in  many  lizards  and  especially  in  Sphenodon  it  penetrates  the  roof  of  the  skull  and 


158 


COMPARA.TIVE   MORPHOLOGY   OF   VERTEBRATES. 


forms  a  well-developed  eye  (fig.  151),  lying  Just  beneath  the  skin  and  connected 
with  the  brain  by  more  or  less  rudimentary  nerves.  In  some  the  pinealis  also 
shows  eye-like  features. 

The  optic  lobes  are  distinct  from  each  other.  The  cerebellum  is  usually 
small  (fig.  159),  but  in  the  crocodilia  (fig.  151),  it  attains  considerable  size.  In 
all  reptiles  there  is  a  thicker  central  portion  and  thinner  lateral  parts,  an  approach 
to  the  differentiation  into  vermis  and  flocculi  found  in  birds.  There  are  no 
special  features  in  the  medulla  calling  for  notice. 

AVES. — ^The  bird's  brain  (fig.  161)  is  short,  broad  and  highly  specialized.  The 
smooth  cerebral  hemispheres  are  large,  their  size  being  due  more  to  the  enormous 
corpora  striata  than  to  enlargement  of  the  pallium,  which  is  comparatively  thin, 
while  the  olfactory  lobes  are  very  slightly  developed,  in  correlation  with  the  deficient 
powers  of  smell.     The  large  cerebellum  extends  forward  between  the  hinder  ends 


C 


A 


Fig.  161. — Brain  of  golden  eagle,  Aquila  chryscBtos,  after  Herrick.     c,  cerebrum;  cl,  cere- 
bellum;/, flocculus;  mo,  medulla  oblongata;  ol,  optic  lobes;  an,  optic  nerve. 

of  the  cerebrum,  thus  forcing  the  optic  lobes  into  a  lateral  position  and  completely 
covering  the  'twixt-brain.  The  epiphysial  structures  are  large  but  rudimentary  in 
character,  the  pinealis  extending  up  in  the  angle  between  cerebrum  and  cere- 
bellum. Below,  the  hypophysis  completely  hides  the  infundibulum.  The  large 
cerebellum  has  its  median  portion  transversely  furrowed,  this  constituting  the 
vermis,  while  the  smaller  lateral  lobes,  which  vary  in  extent,  form  the  flocculi. 
The  myelencephalon  is  very  short  and  the  fossa  rhomboidea  is  covered  by  the 
cerebellum. 

MAMMALS. — The  brain  in  the  mammals  becomes  exceedingly  complex.  Only 
the  most  important  features  and  those  of  general  occurrence  will  be  noted  here. 
Most  marked  are  the  large  size  of  the  cerebellum  and  the  still  greater  development 
of  the  cerebrum,  correlated  with  the  great  increase  in  mental  powers.     The  cere- 


BIL\IN.  159 

brum  covers  the  di-  and  mesencephalon,  and  in  the  primates  even  the  whole  of  the 
cerebellum  from  above.  This  increase  of  the  cerebrum  is  largely  an  increase  of 
the  nervous  matter  of  the  pallium,  a  portion — the  neopallium — developing  on  the 
lateral  side  of  each  hemisphere  between  the  hippocampus  and  the  basal  structures 
(pyriform  lobes).  This  increase  in  cerebrum  is  limited  in  forward  and  backward 
growth  by  the  limitations  of  skull  development.     Hence  it  overlaps  the  olfactory 


T^ip 


\ 


Fig.  162. — ^Ventral  surface  of  brain  of  Ornithorhynchus,  after  G.  Elliot  Smith,  bo, 
bulbus  olfactorius;  c^,  first  cervical  nerve;  cl,  cerebellum;  cm,  corpus  mamillare;/,  floc- 
culus; Ip,  lobus  pyrifonnis;  op,  olfactory  peduncle;  rf,  rhinal  fissure;  ic,  tuber  dnereimi;  to, 
olfactory  tubercle;  tV,  tuberculum  quinti;  Vm,  Vmd,  Vmx,  motor  root  and  maxillaris  and 
mandibularis  roots  of  trigeminal  nerve;  I-XII,  cranial  nerves.     See  ako  fig.  152. 

lobes  in  front,  so  that  they  appear  to  rise  from  its  ventral  surface,  while  behind  it 
extends  backward,  then  turns  downward  and  lastly  extends  forward  along  the  sides 
of  the  mid-  and  'twixt-brains,  even  overlapping  a  part  of  the  cerebrum  itself.  In 
this  way  the  cerebrum  becomes  marked  off  into  a  series  of  regions  called  the  frontal 
lobes  in  front,  the  parietal  above,  the  occipital  behind,  while  the  reflexed  ventral 
portion  of  either  side  makes  a  temporal  lobe. 

This  folding  and  overgrowth  causes  grooves  or  fissures  in  the  surface  of  the 
cerebrum,  the  most  constant  being  a  rhinal  fissure  between  olfactory  and  frontal 
lobes,  a  Sylvian  fissure  between  the  temporal  lobe  and  the  lower  surface  of  the 


i6o 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


cerebrum  against  which  it  is  folded.  In  the  bottom  of  the  Sylvian  fissure  is  a  part 
of  the  side  wall  of  the  cerebrum  which  has  received  the  |j^ame  of  insula  (island  of 
Riel),  while  a  hippocampal  fissure  causes  the  hippocampus  to  appear  as  a  pro- 
nounced swelling  on  the  floor  of  each  ventricle.  In  the  lower  mammals  these  are 
the  only  fissures  present,  the  rest  of  the  cerebral  surface  being  smooth.  In  the 
higher  mammals  other  grooves  (sulci)  separating  convolutions  (gyri)  appear. 
These  convolutions  increase  the  extent  of  cerebral  surface  and  as  a  consequence 
they  permit  of  more  cortical  gray  matter  upon  which  mentality  depends.  The 
number  of  gyri  increases  in  the  primates  and  reaches  its  extreme  in  man.  The 
folding  of  the  cerebrum  also  affects  the  cavities  of  the  cerebrum  as  well  as  the  course 
of  the  fibre  tracts,  especially  of  the  fornix  which  becomes  greatly  bent  on  itself. 
In  the  ventricles  distinct  regions  or  'horns'  are  recognized,  an  anterior  comu  in 


Fig.  163.  Fig.  164. 

Fig.  163. — Brain  of  Chrysothryx  sciureus,  after  Weber.  /,  frontal  lobe;  i,  interparietal 
fissure;  0,  occipital  lobe;  p,  parietal  lobe;  s,  Sylvian  fissure;  t,  temporal  lobe;  ts,  sulcus 
temporalis. 

Fig.  164. — Brain  of  Manis  javanica,  after  Weber,  ch,  cerebellar  hemispheres;  h^ 
hippocampal  lobe;  0,  olfactory  lobe;  pSy  presylvian  fissure;  s,  Sylvian  fissure;  ss,  sulcus 
sagittalis;  v,  vermis;  II,  optic  nerve. 

the  frontal  lobe,  a  posterior  in  the  occipital  lobe  and  an  inferior  comu  in  the 
temporal  lobe.  Associated  with  the  cortical  gray  matter  are  nerve  fibres  (compara- 
tively few  in  the  lower,  extremely  numerous  in  the  higher  mammals)  which  form 
a  corona  radiata  and  connect  the  cortex  with  the  more  posterior  regions  of  the 
brain.  In  the  non-placental  mammals  the  anterior  commissure  is  very  large, 
forming  the  chief  association  tract  between  the  two  hemispheres,  but  in  the  higher 
groups  the  corpus  callosum  becomes  greatly  developed  and  largely  replaces  it. 

The  diencephalon  is  greatly  reduced,  the  hypophysis  and  infundibulum  being 
small,  the  latter  showing  traces  of  the  saccus  vasculosus  and  inferior  lobes  so 
prominent  in  the  lower  vertebrates.  The  parietal  organ  is  lacking,  but  the  pinealis 
is  relatively  large.  It  is  separated  from  the  roof  of  the  skull  by  the  occipital 
lobes  of  the  cerebrum.  It  is  connected  with  the  roof  of  the  brain  by  two  bands 
or  peduncles  and  its  cavity  contains  a  quantity  of  so-called  'brain  sand.'  A 
transverse  groove  divides  the  optic  lobes  so  that  they  consist  of  four  lobes  (corpora 
quadrigemina). 


SPINAL   NERVES.  l6l 

The  cerebellum  is  divided  into  a  median  vermis  and  a  pair  of  lateral  portions, 
each  consisting  of  a  large  cerebellar  hemisphere,  ventral  (morphologically  lateral) 
to  which  is  a  flocculus  (fig.  162),  homologous  to  that  of  the  sauropsida.  The 
surface  of  the  hemispheres  is  convoluted  and  this  results  in  the  arrangement  of 
the  white  and  gray  matter  in  such  a  way  that  they  have  a  markedly  dendritic  ap- 
pearance (arbor  vitae,  fig.  152)  when  seen  in  longitudinal  section.  The  pons, 
characteristic  of  the  mammalian  brain,  has  already  been  mentioned  (p.  150). 

THE  PERIPHERAL  NERVOUS  SYSTEM. 

The  Spinal  Nerves. 

The  spinal  nerves  are  metameric  structures,  connected  with  the 
spinal  cord  by  two  separate  portions  or  roots  which  differ  greatly  from 
each  other  in  development,  structure  and  function.  At  the  time  of 
the  closure  of  the  neural  tube  a  band  of  cells  occurs  on  either  side  of  the 
neural  plate  at  the  junction  of  neural  and  epidermal  areas.  With  the 
closure  of  the  tube  these  form  two  bands,  the  neural  crests,  one  on 
either  side  of  the  dorsal  surface  of  the  cord  (fig.  144).  By  unequal 
growth  each  crest  soon  develops  a  series  of  metameric  enlargements,  the 
portions  of  the  crest  between  these  gradually  disappearing,  while  the  en- 
largements form  the  ganglia  of  the  dorsal  roots  of  the  nerves.  Each  of 
its  cells,  like  those  of  the  cord,  sends  out  processes,  one  of  which  grows 
medially  and  enters  the  cord  in  the  region  of  the  posterior  cornu,  while 
the  other  extends  peripherally  to  the  skin  or  viscera,  these  processes 
constituting  the  dorsal  root  of  the  nen^e,  the  ganglion  forming  an 
enlargement  upon  it,  near  its  connection  with  the  cord.  The  other  or 
ventral  root  is  formed  by  fibres  which  grow  out  in  a  similar  way  from 
cells  in  the  ventral  horn  of  the  cord  itself  and  leave  it  between  the  an- 
terior and  lateral  columns,  to  extend  to  the  muscles,  glands,  etc.  As  the 
ganglion  cells  are  inside  the  cord,  there  is  no  ganglion  on  the  ventral 
root.  Except  in  the  cyclostomes  the  dorsal  and  ventral  roots  unite 
soon  after  leaving  the  cord,  the  combined  trunk  being  a  t}^pical  spinal 
nerve  (figs.  145,  166). 

Physiologically  the  roots  differ  in  that  the  dorsal  roots  are  mainly 
composed  of  sensory  fibres,  while  the  ventral  roots  contain  only  motor 
fibres.  That  is,  on  stimulation  of  the  parts  to  which  they  are  distrib- 
uted the  dorsal  roots  and  their  fibres  carry  nervous  impulses  to  the  cord 
— they  are  afferent — while  the  impulses  in  the  ventral  roots  are  carried 
in  the  opposite  direction  by  efferent  fibres.  In  their  case  stimulation 
arises  in  the  central  nervous  system  and  the  impulse  is  carried  outward 


l62 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


to  the  parts  to  which  the  fibres  are  distributed,  causing  these  to  act — 
muscles  to  contract,  glands  to  secrete,  etc.  Hence  the  ventral  roots  are 
called  motor  roots.  Their  fibres  are  without  sensory  functions,  while 
sensory  fibres  are  equally  unable  to  cause  action  in  any  peripheral 
part  (Bell's  law). 

After  a  longer  or  shorter  course,  each  spinal  nerve,  formed  by  the 
union  of  dorsal  and  ventral  roots,  divides  into  three  branches,  each  of 
which  receives  both  sensory  and  motor  fibres.     These  are  known  as 

^^  I 


JT 


Fig.  165. — A,  diagram  of  collector  nerve;  B,  of  a  nerve  plexus,  after  Braus;  C,  branchial 
plexus  of  Salamandra  maculata,  after  Fiirb ringer. 

the  ramus  dorsalis,  ramus  ventralis  and  ramus  visceralis  or  in- 
testinalis.  The  first  goes  to  the  skin  and  muscles  of  the  dorsal  region; 
the  second  to  those  of  the  sides  and  ventral  parts  of  the  body;  while  the 
visceral  branch  descends  to  the  roof  of  the  coelom,  near  the  insertion  of 
the  mesentery,  where  it  connects  with  the  sympathetic  nervous  system 
to  be  described  below  (fig.  166). 

Recent  physiological  and  histological  analysis  shows  the  existence 
of  two  groups  of  nervous  elements  in  both  sensory  and  motor  nerves. 
There  are  somatic  sensory  and  motor  fibres,  distributed  to  the  skin 
and  most  of  the  external  sense  organs  and  to  the  voluntary  muscles,  and 


SYMPATHETIC   SYSTEM.  1 63 

there  are  also  visceral  fibres  of  both  kinds,  supplying  the  viscera 
(alimentary  canal,  excretory  and  reproductive  organs)  and  the  circula- 
tory system.  The  dorsal  and  ventral  rami  contain  mostly  somatic 
fibres  with  a  few  of  the  visceral  type,  while  the  visceral  rami  are  com- 
posed of  visceral  fibres  alone.  The  farther  subdivision  of  these  nerves 
will  be  considered  later. 

To  the  statement  that  the  dorsal  roots  are  purely  sensory  the  exception  must  be 
made  that  in  the  lower  vertebrates  some  of  the  visceral  motor  fibres,  arising  in 
the  neighborhood  of  the  lateral  cornu,  pass  out  from  the  cord  through  the  dorsal 
root.  In  the  mammals  they  are  said  to  leave  by  the  ventral  roots  like  all  other 
motor  fibres. 

In  the  regions  of  the  appendages  the  spinal  nerves  usually  form 
networks  or  plexuses,  branches  of  a  varying  number  of  ventral  rami 
interlacing  in  a  complicated  manner  before  entering  the  appendage. 
Plexuses  are  poorly  developed  in  the  fishes,  but  here  many  spinal  nerves 
are  united  before  entering  a  limb  by  means  of  a  longitudinal  *col- 
lector'  nerv^e,  there  being  no  exchange  of  fibres  such  as  occurs  in  a 
plexus.  In  the  amphibia  there  are  two  plexuses,  a  cervico-brachial 
near  the  fore  limb,  and  a  lumbo-sacral  for  the  hind  limb.  In  the 
higher  groups  there  may  be  four  plexuses:  cervical,  brachial,  lum- 
bar and  sacral,  the  positions  of  which  are  indicated  by  their  names. 

The  Sympathetic  System. 

The  function  of  the  sympathetic  system  is  the  control  of  the  viscera, 
various  glands,  the  smooth  muscles,  and  through  the  latter,  of  the  size 
of  the  blood-vessels  and  the  supply  of  blood  to  the  various  parts. 
The  system  is  connected  with  the  spinal  nerves  by  the  visceral  rami 
(rami  communicantes)  already  mentioned.  As  has  just  been  said, 
these  visceral  rami  contain  both  motor  and  sensory  fibres.  As  these 
rami  extend  downward  in  their  development,  they  carry  with  them 
ganglion  cells  derived  from  the  ganglia  of  the  dorsal  roots  of  the 
spinal  nerves,  and  these  give  rise  to  the  sympathetic  ganglia.  Of 
these  there  are  three  groups.  Nearest  to  the  spinal  nerves  on  either 
side  are  a  series  of  the  sympathetic  trunk  (chain  ganglia),  usually 
connected  with  each  other  by  a  longitudinal  sympathetic  trunk. 
Nerves  run  from  these  chain  ganglia  to  the  prevertebral  ganglia, 
some  of  which,  like  the  cardiac,  pelvic,  hypogastric  and  solar 
(plexuses)   are  of   considerable  size.     From   these  nerves  go  to    the 


164 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


peripheral  ganglia,  situated  at  various  points  along  the  viscera,  some  at 
some  distance  from  the  sympathetic  centres. 

In  the  sympathetic  system  four  kinds  of  nervous  elements  are  to  be 
distinguished.  The  original  trunk  that  grows  out  (the  ramus  visceralis) 
consists  of  motor  and  sensory  fibres.  The  latter  arise  from  ganglion 
cells  in  the  ganglia  of  the  dorsal  roots.  The  motor  fibres  have  their 
cell  bodies  in  the  cord  at  about  the  level  of  the  lateral  cornu,  and  pass 


visceral  motor 
somatic  motor 
visceral  sensory 
somatic  sensory 
sympathetic 


Fig.  166. — Diagram  of  the  relations  of  the  sympathetic  system,  based  on  Huber.  The 
character  of  the  different  fibres  is  shown  by  conventional  Unes.  bv,  blood-vessel;  eg,  chain 
ganglion;  d,  dorsal  ramus;  dr,  dorsal  root;  g,  gland;  gr,  gray  ramus;  pg,  peripheral  ganglion; 
pvg,  prevertebral  ganglion;  st,  sympathetic  trunk;  v,  ventral  ramus;  vi,  visceral  ramus;  vr, 
ventral  root;  wr,  white  ramus. 


out,  in  the  lower  vertebrates  by  the  dorsal,  in  the  mammals  by  the 
ventral  root.  In  the  sympathetic  system  itself  there  are  sensory  and 
motor  (excitatory)  cells,  derived  from  the  ganglion  cells  carried  down 
by  the  growing  nerves.  These  develop  their  dendrites  and  axons, 
and  some  of  these  run  up  the  rami  communicantes  to  the  dorsal  and 
ventral  rami,  and  follow  along  them  to  the  peripheral  glands  and 
blood-vessels  of  the  body.  Others  grow  into  the  various  viscera.  These 
purely  sympathetic  fibers  are  not  medullated  and  hence  are  gray  in 


CRANIAL  NERVES.  1 65 

color,  and  form  gray  rami  communicantes  for  a  part  of  their  course  to 
the  spinal  nerves. 

The  sympathetic  system  is  best  developed  in  the  trunk,  but  it 
extends  forward  into  the  head,  where  a  series  of  sympathetic  ganglia 
(ciliary,  sphenopalatine,  etc.)  is  connected  with  the  cranial  nerves 
as  far  forward  as  the  fifth.  The  sympathetic  trunk  in  this  region  is 
usually  closely  connected  with  other  nerves,  but  occasionally  (Vidian 
nerve  from  the  sphenopalatine  to  the  facial  ganglion,  Jacobson's 
commissure  from  the  seventh  to  the  ninth,  fig.  170)  it  is  distinct. 

A  few  words  may  be  added  to  this  general  account.  In  the  elas- 
mobranchs  there  is  no  sympathetic  trunk,  this  first  appearing  in  the 
teleosts.  The  system  is  more  highly  developed  in  the  aquatic  than 
in  the  terrestrial  urodeles  or  in  the  anura.  In  the  sauropsida  the 
trunk  is  usually  double  on  either  side  in  the  neck  region,  one  branch 
running  through  the  vertebrarterial  canal  of  the  vertebrae.  In  the 
mammals  the  cervical  part  of  the  trunk  is  usually  closely  associated 
with  the  pneumogastric  nerv^e.  In  the  development  certain  ganglion 
cells  migrate  from  the  developing  sympathetic  system  and  pass  to  various 
parts  of  the  body,  being  usually  closely  associated  with  the  glands  of 
so-called  internal  secretion — hypophysis,  carotid  gland,  suprarenals, 
etc.  They  possess  a  peculiar  ajQ&nity  for  chromic  salts  and  are 
known  as  chromaffine  cells.     Little  is  known  of  their  function. 

The  Cranial  Nerves. 

The  nervxs  which  arise  from  the  brain  and  pass  out  through  the 
foramina  in  the  skull  are  known  as  the  cranial  nerves.  While  in  a 
general  way  they  resemble  the  spinal  nerves,  they  have  been  specialized 
and  modified  in  many  respects  in  correspondence  with  the  specialization 
of  the  head  itself,  some  consisting  of  sensory  fibres  alone,  some  of  only 
motor  fibres,  while  others  are  mixed,  that  is,  contain  both  kinds  of 
fibres.  There  can  also  be  recognized  somatic  and  visceral  nervxs  as  in 
the  trunk,  while  the  somatic  sensory  fibres  may  be  arranged  in  dif- 
ferent groups,  differing  in  their  connexions  inside  the  brain  and  in  the 
sense  organs  to  which  they  are  distributed.  Thus  six  different  kinds  of 
fibres  may  occur  in  the  cranial  nerv^es,  as  follows: 

1.  The  somatic  motor,  which  go  to  the  muscles  derived  from  the 
myotomes  of  the  head. 

2.  The  visceral  motor,  distributed  to  the  muscles  of  the  gill  region, 


1 66 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


Fig.  167. — Brain  and  cranial  nerves  of  Carcharias  littoralis  (Princeton  310) ,  natural  size. 
60,  olfactory  bulb;  6r^-*,  branchial  nerves;  cr,  corpus  restiforme;  d,  diencephalon;  e,  epi- 
physis; ec,  external  canal  of  ear;  er,  external  rectus;  eo,  external  oblique;  Aw,  hyomandibular 
nerve;  /,  lateralis  nerve;  md,  mandibularis  nerve;  ms,  mesencephalon,  also,  maxillaris 
superior;  00,  olfactory  organ;  os,  ophthalmicus  superficialis  nerve;  ot,  olfactory  tract;  pal, 
palatine  nerve;  pc,  posterior  canal;  po,  post-trematic  branch;  pr,  pretrematic  branch;  so, 
superior  oblique;  sr,  superior  rectus,  t,  telencephalon;  m,  utriculus;  v,  visceral  branch  of  X; 
I-X,  cranial  nerves. 


CRANIAL   NERVES.  1 67 

and  their  homologues  in  the  higher  vertebrates,  arising  from  the  lateral 
plate  region  of  the  embryo. 

3.  The  visceral  sensory  nerves  are  connected  inside  the  brain 
with  the  communis  tract  of  the  medulla  (fascicularis  solitarius  of  human 
anatomy),  while  they  terminate  in  special  taste  organs,  usually  within 
the  mouth,  but  in  many  teleostomes  distributed  on  the  sides  of  the  body 
as  well. 

4.  The  general  cutaneous  sensory  nerves,  corresponding  to  the 
somatic  sensory  of  the  trunk.  Internally  they  are  connected  with  the 
dorsal  horns  of  the  spinal  cord  and  the  homologous  parts  of  the  myelen- 
cephalon,  while  distally  they  terminate  either  as  free  nerves  or  in  special 
tactile  organs  in  the  skin. 

5.  The  acustico-lateralis  nerves,  the  centre  of  which  is  in  the 
cerebellum  and  in  the  tuberculum  acusticum  of  the  myelencephalon. 
Distally  the  fibres  terminate  in  peculiar  collections  of  sense  cells  known 
as  sense'  hillocks  or  neuromasts  occurring  in  the  inner  ear  and  in  the 
lateral  line  organs  of  the  ichthyopsida. 

6.  The  nerves  of  special  sense  (olfactory  and  optic). 

The  first  four  of  these  groups  occur  in  the  spinal  nerv^es;  the  last  two 
are  confined  to  the  head.  While  each  spinal  nerve  contains  all  four 
components,  the  same  is  not  true  of  most  of  the  cranial  nen^es,  some 
having  but  a  single  kind  of  fibre.  On  this  and  other  accounts  it  is 
necessary  to  review  each  nerve  in  some  detail.  In  the  lower  verte- 
brates (ichthyopsida)  there  are  ten  of  these  cranial  nerves;  in  the  am- 
niotes  there  are  twelve.  These  are  known  by  both  name  and  number. 
I.  The  Olfactory  Nerve  differs  considerably  in  the  various  groups 
of  vertebrates.  The  term  strictly  includes  only  the  fibres  extending 
between  the  olfactory  lobe  of  the  brain  and  the  olfactory  epithelium, 
the  fibres  terminating  in  the  rhinencephalon  by  dendrites  which,  in- 
terlacing with  dendrites  of  cerebral  neurons,  form  oval  bodies,  the 
glomeruli.  The  olfactory  nerve  differs  from  all  others  in  that  it  arises 
from  cells  of  the  epidermis.  In  some  vertebrates  (elasmobranchs, 
some  teleosts,  ganoids,  snakes,  some  lizards,  fig.  168,  ^),the  nerve  proper 
is  very  short,  while  the  olfactory  lobe  is  developed  into  an  elongate 
structure  in  which  separate  regions  may  be  distinguished,  a  part  of  the 
lobe  remaining  in  connexion  with  the  cerebrum,  next  a  narrower  stalk, 
the  tractus,  and  lastly  a  larger  bulbus  olfactorius,  containing  the 
glomeruli,  close  to  the  nasal  organ.  In  other  vertebrates  (some 
teleosts,  amphibia,  some  lizards,  turtles,  fig.  i68,  B)  the  nerve  is  more 


1 68 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


elongate  and  the  lobe  is  not  differentiated  into  bulb  and  tract.  The 
nerve  in  all  forms  consists  solely  of  special  sensory  fibres,  and  its  ap- 
parent origin  from  either  the  tip  or  the  ventral  surface  of  the  cerebrum 


Fig.  i68. — Diagrams  of  the  different  kinds  of  oKactory  bulb,  tract,  and  nerve,     bo,  ol- 
factory bulb;  ^,  glomeruli;  ol,  olfactory  lobe;  on,  olfactory  nerve;  to,  olfactory  tract. 


Fig.  169. — Brain  and  olfactory  and  {nt)  terminalis  nerves  of  Raia,  after  Locy. 

is  to  be  explained  by  the  varying  development  of  the  hemispheres  as 
given  above  (p.  159). 

In  some  fishes  (several  elasmobranchs,  dipnoi,  Amia)  a  small 
nerve  arises  dorsally  (some  elasmobranchs)  or  ventrally  from  the  cere- 
brum, has  a  distinct  ganglion  and,  following  somewhat  closely  the 


CRANIAL  NERVES.  1 69 

olfactory  nen^e,  is  distributed  to  the  olfactory  epithelium.  Apparently 
the  same  nerve  occurs  in  the  human  embryo  and  it  may  be  looked  for 
elsewhere.  It  is  called  the  terminalis  nerve  and  probably  belongs  to 
the  general  cutaneous  system. 

II.  The  Optic  Nerve  arises  from  the  floor  of  the  diencephalon  and 
extends  to  the  eye  where  it  spreads  over  the  inner  surface  of  the  retina. 
Together  with  the  olfactory  nerv^e  it  is  usually  stated  to  differ  from  the 
other  cranial  nerves  in  being  an  outgrowth  from  the  brain.  In  its 
history,  which  is  closely  connected  with  that  of  the  eye,  there  is  first 
formed  the  optic  stalk  with  the  optic  vesicle  at  its  tip  (see  eye  for  details). 
The  stalk  grows  out  from  the  recessus  opticus  and  hence  is  clearly 
dorsal  in  position.  Soon  after  the  involution  of  the  optic  cup,  nerve 
cells  are  proliferated  from  the  distal  surface  of  the  retina,  which  pass 
through  the  chorioid  fissure  and  along  the  groove  on  the  ventral  side  of 
the  optic  stalk.  These  fibres  and  not  the  cells  of  the  stalk  form  the 
definitive  optic  nerve  of  the  adult,  and  the  cells  from  which  they  arise 
form  the  optic  ganglion,  which,  to  a  certain  extent,  is  comparable  to  the 
ganglion  of  a  dorsal  root.  This  view  also  lessens  the  differences  be- 
tween the  optic  and  other  cranial  nerves,  a  view  which  was  natural 
before  the  history  of  the  nerve  was  known  and  when  it  was  thought 
that  the  stalk  itself  was  transformed  into   the  nerve. 

The  nerve  fibres,  in  their  centripetal  growth,  do  not  stop  on  reaching 
the  diencephalon,  but  continue  across  its  ventral  surface  and  become 
connected  with  the  opposite  side  of  the  brain.  There  is  thus  a  crossing 
or  chiasma  of  the  optic  nerves,  that  from  the  left  eye  going  to  the  right 
side  of  the  brain  and  vice  versa.  In  most  vertebrates  the  chiasma  is 
plainly  seen  from  the  surface,  but  in  cyclostomes  and  dipnoans  it  may 
occur  in  the  substance  of  the  brain  itself.  In  the  lower  vertebrates  the 
chiasma  is  complete  and  the  nerves  from  the  two  sides  may  simply  over- 
lap or  they  may  interlace  with  varying  degrees  of  complexity.  In  the 
mammals,  on  the  other  hand,  the  chiasma  can  be  analyzed  only  by 
microscopic  methods,  so  intimately  are  the  fibres  interwoven,  while  here 
some  of  the  fibres  ('lateral  fibres'),  instead  of  crossing,  enter  the  cor- 
responding side  of  the  brain.  The  internal  connections  of  the  optic 
nerves  are  not  with  the  'twixt-brain,  but  the  fibres,  after  passing  the 
chiasma,  grow  dorsally  and  posteriorly  and  become  connected  with  the 
dorsal  part  of  the  mid-brain,  hence  called  the  optic  lobes. 

There  has  been  described  in  the  embryo  elasmobranch,  under  the  name  thal- 
amic nerve  a  small  strand  arising  between  the  di-  and  mesencephalon.     It  disap- 


lyo  COMPARATIVE    MORPHOLOGY    OF    VERTEBRATES. 

pears  without  leaving  a  trace,  unless  it  contribute  to  the  ciliary  ganglion.     Its  status 
as  a  nerve  is  very  uncertain. 

The  Eye  Muscle  Nerves  (fig.  137). — The  III  (oculomotorius), 
IV  (trochlearis)  and  VI  (abducens)  nerves  are  distributed  to  the 
muscles  which  control  the  movements  of  the  eye  and  hence  are  treated 
together.  The  oculomotor  supplies  the  superior,  inferior  and  internal 
rectus  and  inferior  oblique  muscles;  the  trochlearis  goes  to  the  superior 
oblique,  while  the  abducens  innervates  the  external  rectus  muscle. 


laletaUs 
vibUTol  ynoloT 
visceral  5en*or\j 
^■=«:«=»  QcneroV  cuUjvtus 

Fig.  170. — Diagram  of  branches  and  components  of  the  fifth  or  trigeminal  nerve  in  a 
shark,  gg,  Gasserian  ganglion;  ;',  Jacobson's  commissure,  connecting  with  glossophar}m- 
geal;  md,  mandibularis  nerve;  mx,  maxillaris  nerve;  op,  os,  ophthalmicus  profundus  and 
superficialis  nerves. 

These  peculiarities  of  distribution  are  explained  by  the  development 
of  the  muscles  (p.  128),  the  derivatives  of  each  somite  having  a  common 
nerve  supply.  The  oculomotor  nerve  springs  from  the  ventral  surface 
of  the  mid-brain,  the  fourth  from  the  dorsal  surface  at  the  hinder  margin 
of  the  mesencephalon,  while  the  sixth  comes  from  the  ventral  surface 
of  the  myelencephalon.  Inside  the  brain  the  trochlearis  is  traced  to 
its  nucleus  in  a  ventral  position. 

In  the  majority  of  vertebrates  these  nerves  are  readily  traced  from 
the  brain  to  the  muscles  they  supply,  but  not  infrequently  the  abducens 
(lacking  in  Petromyzon)  is  united  proximally  with  the  fifth  nerve, 
while  in  a  few  forms  the  trochlearis  has  not  been  recognized,  and  it  is 
said  that  in  the  adult  Bdellostoma  all  eye-muscle  nerves  are  lacking. 
The  ciliary  ganglion  is  closely  associated  with  the  oculomotor  nerve. 
All  three  eye-muscle  nerves  belong  to  the  somatic  motor  group. 

V.  The  Trigeminal,  one  of  the  largest  of  the  cranial  nervxs,  arises 


CRANIAL   NERVES.  171 

from  the  anterio-lateral  angle  of  the  myencephalon,  and  its  fibres  pass 
almost  immediately  into  the  semilunar  (Gasserian)  ganglion, 
which  may  lie  either  within  or  without  the  skull.  In  the  higher  verte- 
brates the  nerve  divides  beyond  the  ganglion  into  three  main  trunks — 
ophthalmic,  maxillary,  and  mandibular — whence  the  name.  In 
the  lower  vertebrates  the  maxillary  and  mandibular  pursue  a  common 
course  for  some  distance  before  separating. 

In  the  fishes  the  ophthalmic  is  represented  by  two  branches,  an 
ophthalmicus  superficialis  (not  to  be  confused  with  the  similarly 
named  branch  of  the  seventh  with  which  it  is  closely  associated)  and 
an  ophthalmicus  profundus,  which  passes  between  the  eye  muscles 
on  its  way  to  the  tip  of  the  head.  Both  are  purely  sensory-,  in  most 
vertebrates  general  cutaneous,  but  in  the  teleosts  they,  together  with 
the  maxillary,  supply  also  the  taste  organs  (visceral  sensory)  of  the 
surface  of  the  head.  The  superficialis  innervates  the  skin  above  and 
in  front  of  the  eye;  the  profundus  goes  to  the  eyelids,  conjunctiva, 
snout  and  the  mucous  membrane  of  the  nose,  passing  through  the 
ciliary  ganglion  in  its  course.  In  the  urodeles,  where  the  maxillaris 
is  reduced,  the  profundus  supplies  its  region. 

The  maxillaris,  with  components  similar  to  those  of  the  ophthalmic, 
runs  beneath  the  eye,  passing  the  sphenopalatine  ganglion  (p.  165) 
in  its  course,  supplying  much  the  same  territory  as  the  ophthalmic  and, 
in  addition,  the  roof  of  the  palate  and  the  teeth  of  the  upper  jaw. 

The  mandibularis  ramus  is  a  mixed  nerve.  The  motor  components 
(visceral)  innerv^ate  the  muscles  of  the  jaws,  some  muscles  of  the  floor 
of  the  mouth  and  in  mammals  the  tensor  tympani  muscle.  The  sensory 
component  (general  cutaneous)  divides  into  two  parts,  the  lingualis 
going  to  the  tongue  and  the  mandibularis  to  the  skin  of  the  lower 
jaw,  chin  and  lower  lip,  and  to  the  lower  teeth.  In  mammals  there  is 
a  weak  auricularis  superficialis  nerve  arising  from  the  mandibularis 
and  going  to  the  temporal  region  and  to  the  conch  of  the  ear. 

Two  ganglia  of  the  sympathetic  system  are  associated  with  the  trigeminal :  the 
otic  ganglion  near  the  exit  from  the  skull  and  the  submaxillary  where  the  lingualis 
bends  to  enter  the  tongue.  From  the  otic  a  trunk  (Jacobson*s  commissure,  p.  165) 
runs  back  to  connect  \\-ith  the  ninth  nerve. 

The  fifth  nerve  is  usually  compared  to  a  post-otic  nerve  (vide  infra)  in  that  the 
mouth  is  regarded  as  the  homologue  of  a  pair  of  gill  clefts,  the  maxillary  being 
the  pre-  and  the  mandibularis  the  post-trematic  nerves  (see  nerve  IX,  below). 
The  homologies  of  the  ophthalmicus  are  less  certain.     Some  facts  seem  to  point 


172 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


to  the  fifth  being  a  compound  nerve,  this  branch  being  the  remnant  of  a  somite 
otherwise  lost.  Others  would  view  the  ophthalmic  as  the  representative  of  the 
dorsal  ramus  of  a  spinal  nerve. 

VII.  The  Facial  Nerve,  the  hindmost  of  the  preotic  nerves,  differs 
greatly  in  the  branchiate  and  the  pulmonate  vertebrates.  In  all  there 
is  a  close  association  with  the  eighth  nerve,  and  in  the  anura,  some 
teleosts  and  ganoids,  and  in  the  holocephals  the  fifth  and  the  seventh 
are  so  closely  related  that  their  ganglia  are  fused. 


Fig.  171. — Dagram  of  seventh  (facial)  nerve;  for  components  see  fig.  170.  b,  buc- 
calis  nerve;  d,  chorda  tympani;  gg,  geniculate  ganglion;  h,  hyoid  nerve;  hm,  hyomandib- 
ular  nerve;  Ig,  lateralis  ganglion;  mxe,  maxillaris  externus  nerve;  os,  ophthalmicus 
superfidalis  nerve;  pal,  palatine  nerve;  sp,  spiracle. 

In  the  aquatic  ichthyopsida  the  several  roots  by  which  the  seventh 
nerve  leaves  the  medulla  unite  in  a  compound  ganglion,  the  upper 
element  being  the  ganglion  of  the  lateralis  component,  the  lower  the 
geniculate,  the  true  ganglion  of  the  seventh  nerve.  Beyond  the 
ganglion  the  nerve  divides  into  five  trunks,  as  follows : 

A.  The  ophthalmicus  superficialis,  which  runs  forward  near 
the  dorsal  surface  of  the  head;  B.  the  buccalis,  which  courses  nearly 
parallel  to  the  maxillaris  of  the  trigeminal  and  is  often  bound  up  with  it; 
C.  the  mandibularis  externus,  which  usually  divides  into  two  branches, 
usually  follows  much  the  same  course  as  the  mandibularis  trigemini, 
and  supplies  the  lower  jaw  and  the  spiracular  and  hyoid  region;  D. 
the  palatinus  which  goes  to  the  mucoUs  membrane  of  the  oral  cavity, 
E.  the  hyoideus  (usually  united  with  the  mandibularis  for  some  distance 
as  a  hyomandibular  nerve),  which  goes  ventrally  and  supplies  the 
mucosa  of  the  mouth  and  the  muscles  of  the  hyoid  region.  In  cases, 
like  many  elasmobranchs,  where  a  spiracle  is  present,  the  hyomandib- 


CRANIAL  NERVES.  1 73 

ularis  passes  behind  it  and  hence  is  a  post-trematic  ramus.  In  some 
cases  a  small  twig  bends  down  from  the  palatine  and  represents  the 
pretrematic  branch. 

Three  of  these — ophthalmicus  superficialis,  buccalis  and  mandib- 
ularis  externus — belong  to  the  lateralis  system,  which  is  unrepre- 
sented in  the  spinal  nerves.  This  has  its  own  ganglion,  which  may  unite 
with  geniculate  or  semilunar,  and  it  supplies  the  lateral  line  system  of 
the  head  (see  sense  organs).  The  superficial  ophthalmic  innervates  the 
supraorbital  line  of  these  organs,  and  in  the  elasmobranchs,  breaks 
up  distally  to  go  to  the  modified  organs  (ampullae  of  Savi  and  Loren- 
zini)  at  the  tip  of  the  snout.  In  the  same  way  the  buccalis  supplies 
the  infraorbital  line  and  the  mandibularis  externus  those  of  the  lower 
jaw  and  the  hyoid  and  spiracular  regions. 

As  there  are  no  myotomic  muscles  in  the  region  supplied  by  the 
facial  nerve,  there  are  no  somatic  motor  components  The  general 
cutaneous  elements  run  in  the  hyoideus  to  the  skin  of  the  hyoiji  region, 
but  in  other  vertebrates  this  territory  is  supplied  by  branches  of  the  fifth, 
which  spread  to  the  operculum  and  to  the  dorsal  surface  of  the  head. 
The  visceral  motor  components  occur  in  the  hyoid  nerve  and,  in  the 
aquatic  forms,  they  supply  the  muscles  of  the  hyoid  region  and  the 
posterior  belly  of  the  depressor  mandibulae.  In  the  mammals,  with 
the  development  of  the  muscles  of  expression  (p.  134),  the  same 
branch  (known  in  human  anatomy  as  the  main  branch  of  the  facial)  has 
a  much  greater  extension,  the  result  of  the  migration  of  the  muscles 
from  the  hyoid  region  to  their  definitive  position. 

The  geniculate  ganglion  belongs  to  the  visceral  sensory  system, 
the  fibres  of  which  run  in  the  hyoid  and  palatine  nerves  to  reach  the 
sense  organs  in  the  oral  cavity  and  in  the  spiracular  gill  when  this  is 
present.  In  those  fishes  where  there  are  taste  organs  on  the  outer 
surface  of  the  body  there  is  a  *nerve  of  Weber'  (ramus  lateralis  ac- 
cessorius)  which  goes  to  the  dorsal  surface  of  the  head  and  then  to  back, 
fins  and  tail,  wherever  the  gustatory  organs  occur.  It  is  frequently 
accompanied  by  fibres  of  the  tenth  nerve.  In  the  mammals  the  visce- 
ral sensory  fibres  occur  in  the  main  trunk  of  the  seventh,  in  the  great 
superficial  petrosal  and  in  the  chorda  tympani  nerves.  This  last  is  a 
post-trematic  nerve  which  passes  through  the  middle  ear  and  thence  on 
the  medial  side  of  the  lower  jaw  to  join  the  lingualis. 

In  the  adult  anura  and  in  the  amniotes,  where  gills  and  lateral  line 
organs  are  lacking,  the  facialis  undergoes  a  corresponding  reduction, 


174 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES 


the  lateralis  nerves  being  lost,  while,  as  stated  above,  the  motor  por- 
tions are  increased  in  the  mammals,  in  correlation  with  the  greater 
development  of  the  facial  muscles. 


Fig.  172. — ^Ventral  view  of  brain  and  cranial  nerves  of  Iguana,  after  Fischer.  I-XII, 
cranial  nerves;  1-3,  first  three  cervical  nerves;  gp,  petrosal  ganglion;  i,  Jacobson's  commis- 
sure; h,  hypoglossal;  n,  nasalis  ramus  of  V;  rf,  ramus  frontalis  of  V;  sy,  sympathetic. 

VIII.  The  Acustic  (Auditory)  Nerve  is  closely  associated  with  the 
seventh,  but  microscopic  analysis  shows  that  it  has  its  own  roots  and 
ganglion.  It  is  purely  sensory,  its  branches  going  to  the  sensory  areas 
of  the  inner  ear.  Its  connections  inside  the  brain  and  the  development 
of  the  ear  itself  (see  sense  organs)  show  that  the  nerve  belongs  to  the 
lateralis  system,  the  ear  being  a  group  of  modified  lateral  line  organs. 
Beyond  the  ganglion  the  nerve  divides  into  a  vestibular  branch, 
supplying  the  utriculus  and  semicircular  canals,  and  a  cochlear  branch, 
going  to  the  lagena  and  to  its  homologue  in  the  mammals,  the  cochlea. 


CRANIAL   NERVES. 


175 


IX.  The  Glossopharyngeal,  the  first  of  the  post-otic  nerves,  has  its 
typical  development  in  the  branchiate  vertebrates.  Its  roots,  both 
motor  and  sensory,  pass  into  the  petrosal  ganglion,  beyond  which  a 
dorsal  ramus  is  given  off  to  the  top  of  the  head,  while  the  main  trunk, 
passing  outward  and  backward,  leaves  the  skull,  either  by  its  own  fora- 
men (most  branchiates)  or  together  with  the  tenth  nerve  (anura  and 
amniotes) .  It  divides,  just  above  the  first  gill  cleft,  into  pre-  and  post- 
trematic  nerves,  which  run  in  the  anterior  and  posterior  walls  of  the 
cleft  to  the  ventral  wall  of  the  pharynx,  the  pretrematic  giving  off  a 
nerv-e  to  the  mucous  membrane  of  the  palate.     Ninth  and  tenth  nerves 


Fig.  173. — Diagram  of  ninth  (glossophan-ngeal)  and  tenth  (vagus)  nerves  of  a  shark; 
for  components  see  fig.  170.  d,  dorsal  ramus;  g,  gastric  nene;  h,  to  heart;  ;,  Jacobson's 
commissure;  /,  lateralis  nerve;  Ig,  lateralis  ganglion;  po,  pr,  post-  and  pretrematic  branches; 
5/>,  spiracle;  st,  to  supratemporal  lateral  line  organs. 

are  usually  closely  associated  (their  ganglia  may  fuse) ,  while  ninth  and 
fifth  are  frequently  connected  by  Jacobson's  commissure  and  the  pala- 
tine branch  may  connect  with  the  geniculate  ganglion.  The  glos- 
sopharyngeal may  contain  three  kinds  of  components,  the  somatic 
motor  fibres  being  absent  because  of  the  failure  of  the  myotomic  muscles 
to  develop.  The  general  cutaneous  is  usually  represented  by  the  dorsal 
ramus  alone,  but  in  Petromyzon  its  fibres  reach  the  ventral  skin  through 
the  post-trematic  nervx.  The  visceral  sensory  fibres  reach  the  taste 
organs  by  way  of  the  pharyngeal  and  lingual  nerves,  while  the  visceral 
motor  elements  go  to  the  muscles  of  the  gill  region  by  way  of  the  post- 
trematic  ramus. 

X.  The  Vagus  is  a  complex  of  nen^es,  each  similar  to  the  ninth, 
with  the  addition,  in  the  branchiates,  of  lateralis  components,  and  sup- 
plying in  them  the  remaining  gill  clefts.     Numerous  rootlets  pass  from 


176 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


the  medulla  to  the  ganglion  jugularis,  beyond  which  the  dorsal  rami 
arise  and  then  the  main  trunk  runs  backward,  giving  off  as  many 
branchial  nerves  as  there  are  gill  clefts,  each  with  an  epibranchial 
ganglion  and  each  dividing  into  pre-  and  post-trematic  rami.  To 
this  extent  the  tenth  is  a  polymeric  nerve  with  coalesced  proximal 
portions. 

Near  the  last  cleft  the  main  trunk  divides  into  two  nerves.  One  of 
these,  the  ramus  lateralis,  continues  back,  just  beneath  the  skin,  to 
innervate  the  lateral  line  organs  of  the  trunk  and  tail.     The  other,  the 


Fig.  174. — Diagram  of  cranial  nerves  of  a  cat,  the  lower  jaw  reflected,  after  Mivart. 
II-XII,  cranial  nerves;  c/,  chorda  tympani;  d,  dentary  nerve;  g,  Gasserian  ganglion;  io, 
infraorbital  nerve;  /,  lingual  nerve;  li,  Is,  laryngeus  inferior  and  superior;  md,  mandibularis 
nerve;  mx,  maxillaris  nerve;  o,  ophthalmic  nerve;  t,  tongue. 

ramus  intestinalis,  goes  inward  and  backward  to  supply  the  oesoph- 
agus, stomach,  heart  and  other  viscera  (in  air-breathing  vertebrates  the 
lungs  also,  whence  the  name  pnexmaogastric  nerve).  In  the  dorsal 
rami  and  the  branchial  nerves  the  components  are  about  the  same  as  in 
the  ninth  nerve.  The  most  caudal  of  the  motor  roots  of  the  vagus 
furnish  visceral  motor  fibres  which  go  to  some  of  the  muscles  connected 
with  the  pectoralarch  and  appendages,  while  others  pass,  by  way  of  the 
intestinalis,  to  the  viscera.  In  the  same  way  visceral  sensory  fibres  go 
through  the  same  nerve  to  the  taste  buds  of  the  pharynx,  and  in  the 


SENSORY    ORGANS.  177 

higher  vertebrates  the  same  components  occur  in  the  pharyngeal, 
laryngeal,  oesophageal,  and  gastric  branches  of  the  intestinalis.  The 
distribution  of  the  vagus  shows  that  the  parts  supplied  are  to  be  re- 
garded as  morphologically  derived  from  the  head,  though  (heart, 
lungs  and  stomach)  they  may  be  far  removed  from  it  in  the  adult. 
Although  details  have  been  mentioned,  some  differences  between 
air-  and  water-breathing  vertebrates  may  be  summarized.  The  lateral 
line  organs  are  associated  with  an  aquatic  life,  occurring  in  the  branchiate 
forms,  even  of  the  amphibia.  With  the  assumption  of  a  pulmonate 
respiration  the  lateral  line  organs  are  lost  and  with  them  go  the  lateralis 
elements  of  the  seventh  and  tenth  nerves.  In  the  amniotes  neither  the 
organs  nor  the  nerves  appear,  even  in  development.  Also  the  loss  of 
gills,  and  the  closure  of  the  clefts  results  in  a  modification  of  the  nerves 
of  the  ventral  regions. 

XI.  The  Accessory  Nerve  appears  as  a  distinct  nerve  in  the  am- 
niotes, though  traces  of  it  appear  in  the  ichthyopsida  where  the  poster- 
ior roots  of  the  vagus  furnish  fibres  which  go  to  muscles  in  the  pectoral 
region.  In  the  amniotes  the  number  of  these  roots  is  increased  (up  to 
seven  in  mammals) ,  the  additions  being  made  to  the  posterior  end  of  the 
series.  The  fibres  run  forward  between  the  dorsal  and  ventral  roots  of 
the  cervical  nerves  and  unite  to  form  a  trunk,  distinct  from  the  vagus, 
which  bends  back  to  supply  muscles  connected  with  the  pectoral  arch. 
The  components  of  this  accessory  nerve  belong  to  the  visceral  motor 
system,  and  the  explanation  of  muscles  connected  with  locomotion  being 
supplied  by  visceral  ner\^es  is  not  easy. 

XII.  The  Hypoglossal  Nerve  of  the  adult  contains  only  somatic 
motor  fibres,  but  in  the  young  of  several  forms,  both  anmiote  and 
ichthyopsidan,  two  or  more  ganglionated  roots  are  formed  which  soon 
disappear.  The  roots  of  the  nen^e  lie  at  the  junction  of  brain  and 
spinal  cord,  and  hence  the  nerve  lies  outside  the  skull  in  the  lower,  in- 
side it  in  the  higher  forms.  The  nerve  contributes  to  the  innervation 
of  the  tongue,  the  trunk  and  the  brachial  plexus  in  the  lower  vertebrates, 
while  in  the  higher  groups  it  is  more  restricted  to  the  tongue  and  the 
sternohyoid  muscle. 

THE  SENSORY  ORGANS. 

The  sensory  organs  are  to  receive  information  from  without  and 
to  transform  it  into  stimuli  to  be  carried  by  the  nerves  to  the  ganglia, 
usually  those  of  the  central  nervous  system.     This  information  varies 


178 


COMPARATIVE  MORPHOLOGY  OF. VERTEBRATES. 


in  character  and  the  organs  consequently  differ  in  structure  according 
to  the  impressions  they  are  to  receive. 

With  very  few  exceptions  the  characteristic  portions  of  the  organs, 
the  sensory  cells,  arise  from  the  ectoderm,  but  accessory  parts,  chiefly 
of  mesodermal  origin,  may  be  so  abundant  as  to  form  the  bulk  of  the 
organ.  In  some  cases  the  organs  may  remain  in  connexion  with  the 
surface  of  the  body  (the  parent  ectoderm)  throughout  life,  but  frequently 
they  sink  to  a  deeper  position  and  become  surrounded  with  a  protective 
sense  capsule,  while  those  connected  with  the  sympathetic  system  may 
be  scattered  throughout  almost  the  entire  body. 


B 


Fig.  175. — Free  nerve  termina- 
tions in  the  skin  of  Salamandra, 
freely  after  Retzius. 


Fig.  176. — Sensory  cells,  after  Fiirbringsr. 
a,  crista  cell  of  ear;  b,  rod  cell  of  eye;  c,  ol- 
factory cell. 


The  recipient  structures  may  be  of  two  kinds.  In  the  one  (fig. 
175)  the  ends  of  the  nerve  receive  the  impressions  from  without,  often 
aided  by  various  accessory  structures.  In  the  other  there  are  specialized 
sense  cells  (fig.  176),  the  peripheral  ends  of  which  bear  different  kinds 
of  cuticular  percipient  parts — hairs,  bristles,  rods,  cones,  etc. — while 
the  basal  ends  of  the  cells  are  connected  with  the  terminations  of  nerve 
cells  which  act  as  the  conducting  elements.  The  distinction  between 
the  two  is  one  of  convenience  rather  than  one  of  physiological  or  mor- 
phological importance,  for  the  'nerves'  of  the  first  are  in  reality  but 
the  prolongations  of  sensory  cells. 


Nerve-end  Apparatus. 

In  many  cases — skin,  alimentary  tract,  muscles,  etc. — the  ends  of 
the  sensory  nerves  lose  their  medullary  sheath  and  break  up  into  fine 
fibrillae  which  terminate,  without  special  accessory  structures,  among  the 
cells  of  the  tissue  to  which  they  are  distributed  (free  nerve  termina- 
tions).    On  the  other  hand,  there  are  numerous  end  organs,  espe- 


SENSORY    ORGANS. 


179 


cially  among  the  terrestrial  vertebrates,  in  which  accessory  parts  are 
present.  For  details  of  these  reference  must  be  made  to  histological 
text-books;  only  a  mention  of  some  of  the  kinds  can  be  made  here. 
In  the  simple  tactile  corpuscle  the  nerve  terminates  with  a  cup 
in  which  is  seated  a  lenticular  tactile  cell  (fig.  177,  A).  Somewhat 
allied  are  Grandry's  (MerkePs)  corpuscles  in  which  two  or  more 
tactile  cells  are  enclosed  in  a  connective  tissue  sheath,  while  the  nervx, 
losing  its  medullary  sheath  as  it  reaches  the  capsule,  expands  into 
plates  which  are  inserted  between  each  two  tactile  cells  (fig.  177,  ^). 


Fig 


-A,  tactile  corpuscle; 
B,  Grandry's  corpuscle. 


Fig,  178. — Vater-Pacinian  corpuscle. 


In  another  series  of  sensory  structures  the  end  of  the  nerv-e  is  club- 
shaped  and  is  surrounded  by  a  connective-tissue  sheath,  either  simple 
(cylindrical  corpuscles),  or  in  Pacini*s  (Vater's,  fig.  178)  and 
Herbst's  corpuscles,  the  sheath  is  formed  of  layers  of  cells,  recalling 
the  coats  of  an  onion,  while  immediately  around  the  club  is  a  layer 
of  cubical  cells.  Still  another  variant  is  found  in  Krausse*s  (corpus- 
culiun  bulboideiun)  and  Meissner*s  corpuscles,  where  the  nerv^e, 
on  entering  the  corpuscle,  breaks  up  into  numerous  branches  which 
surround  an  axial  core  of  large  cells. 

It  is  impossible  at  present  to  state  with  certainty  the  function  of 
each  of  these  and  other  nerve-end  apparatuses  and  to  say  which  are 
connected  with  the  different  senses — tactile,  pressure,  pain,  heat  and 
cold,  muscular,  etc. — which  are  commonly  confused  under  the  term 
'touch.' 


Lateral  Line  Organs. 

The  lateral  line  organs  occur  only  in  the  ichthyopsida  and  here 
only  during  the  branchiate  stages.  They  arise  as  thickenings  of  the 
ectoderm  on  either  side  of  the  head  in  the  neighborhood  of  the  ear. 
From  here  the  thickenings  extend  in  definite  lines  which  determine  the 


i8o 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


series  of  organs  in  the  adult.  At  points  on  these  lines  the  sensory- 
areas  are  developed  by  the  differentiation  of  two  kind  of  cells,  the 
supporting  cells  which  extend  through  the  epidermis  from  the  corium 
to  the  free  surface,  and  the  sensory  cells  which  reach  from  the  surface 
only  part  way  to  the  base.  The  latter  are  pear-shaped  and  bear 
cuticular  hairs  or  bristles  on  their  free  ends  (fig.  179),  while  the  deeper 
ends  are  embraced  by  the  non-medullated  fibrils  of  the  lateralis  system 
of  nerves,  which  follow  the  lines  of  organs,  and  in  development  keep 
pace  with  their  extension.  These  sensory  areas  are  the  nerve  hillocks 
or  neuromasts  alreadv  referred  to. 


Fig.  179.  Fig.  180. 

Fig.  179. — Sense  organ  of  lateral  line  of  Diemyctylus  (aquatic  form)  freely  after  Kings- 
bury, C,  cone  cells;  s,  spindle  cells. 

Fig.  180. — Developing  lateral  line  organ  on  one  side  of  head  of  Amia,  showing  method 
of  closure  of  grooves  to  canals,  after  Allis.  an,  anterior  naris;  io,  so,  infra-  and  supraorbital 
lines;  pn,  posterior  naris. 


In  the  cyclostomes  and  aquatic  amphibia  each  sensory  patch 
sinks  into  a  separate  pit  (fig.  179),  but  in  all  other  itchhyopsida  the 
lines  of  organs  sink  in  the  same  way,  the  patches  being  connected  by 
grooves.  In  ChimcBra  these  grooves  remain  open,  but  in  all  others  they 
are  closed  except  at  certain  points  where  pores  connect  the  canals 
formed  by  the  closed  grooves  with  the  exterior.  In  this  way  the  sensory 
areas  come  to  lie  in  canals  beneath  the  surface,  water  obtaining  access 
to  them  through  the  pores.  In  many  teleosts  (fig.  181)  the  pores 
pass  through  notches  or  openings  in  the  scales,  while  on  the  head  the 
canals  themselves  frequently  run  through  some  of  the  cranial  bones. 

Of  considerable  morpholggical  importance,  especially  in  connection  with  the 
morphology  of  the  ear,  are  the  facts  that  the  sensory  areas  multiply  by  elongation, 
followed  by  division,  and  that  the  pores  themselves  increase  in  the  same  way 


SENSORY    ORGANS. 


l8l 


(fig.  1 80);  the  pore  elongates  and  then  its  margins  meet  in  the  middle,  thus 
producing  two  pores.  There  has  been  much  discussion  as  to  the  development  of 
the  lateralis  nerves,  especially  that  of  the  trunk,  some  thinking  that  it  increases  by- 
additions  from  the  ectoderm  of  the  skin.  It  appears  more  probable  that  all  of  its 
material  is  derived  from  the  nerve  and  that  there  are  no  additions  from  other  sources. 


Fig.  181. — Stereogram  of  lateral  line  organs  of  a  fish,     c,  lateral  line  canal;  /n,  lateralis 
nerve;  p,  pores  connecting  with  the  exterior;  s,  scales  in  skin;  so,  sense  organs  of  lateral  line. 


Fig.  182 . — Head  of  pollack,  showing  lateral  line  canals  and  nerves  of  the  lateralis  system, 
after  Cole.  Lateralis  nerves  black,  canals  and  brain  dotted.  &,  buccalis  ramus  of  VII 
nerve;  dl,  dorsal  ramus  of  lateralis  of  X  nerve;  h,  hyomandibularis  nerve;  hm,  hyomandib- 
ular  line  of  organs;  io,  infraorbital  Hne;  I,  lateral  line  canal;  n,  nares;  0,  olfactory  lobe;  op, 
operculum;  os,  ophthalmicus  superficialis  nerve;  soc,  conunissure  connecting  lines  of  the 
two  sides;  so,  supraorbital  line  of  organs;  st,  supra  temporal  part  of  lateral  line;  id, 
ventral  ramus  of  lateralis  of  X  nerve;  x,  visceralis  part  of  X  nerve. 

The  distribution  of  these  organs  and  their  canals  varies  considerably. 
The  most  constant  lines  are  the  following  (fig.  182):  A  supraorbital 
line  running  forward  from  the  region  of  the  ear,  above  the  eye,  to  the 


1 82  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

tip  of  the  snout  and  innervated  by  the  ophthalmicus  superficialis 
branch  of  the  seventh  nerve;  an  infraorbital  line  running  in  the  same 
way  beneath  the  eye  and  supplied  by  the  buccalis  nerve;  a  hyomandib- 
ular  line  extending  along  the  lower  jaw  (and  the  operculum  when 
present),  and  innervated  by  the  mandibularis  externus;  and  lastly 
the  lateral  line  proper  (sometimes  double)  which  runs  back  on  either 
side  to  the  tail  and  is  supplied  by  the  lateralis  of  the  tenth  nerve. 
Frequently  the  systems  of  the  two  sides  are  connected  by  a  supra - 
temporal  line  extending  across  the  hinder  part  of  the  skull,  from  one 
side  to  the  other. 

The  lateral  line  organs  appear  in  the  larvae  of  all  amphibia,  but  on 
the  assumption  of  a  terrestrial  life  they  sink  beneath  the  skin  and 
usually  degenerate,  all  traces  of  them  and  the  lateralis  ner\xs  being  lost 
in  the  adult.  In  a  few  cases  {Triton,  Amhly stoma,  etc.)  they  are  said 
not  to  be  entirely  lost,  but  to  reappear  at  the  surface  when  the  animals 
return  to  the  water  for  oviposition.  Various  functions  have  been  assigned 
to  the  lateral  line  organs.  Since  they  contain  much  mucus  they  were 
long  called  slime  organs.  Then  they  were  recognized  as  sensory  and  a 
*  sixth  sense '  was  attributed  to  them.  Recently  it  has  been  made  very 
probable  that  they  are  to  recognize  vibrations  of  a  slow  rate  in  the 
water  and  thus,  among  other  things,  to  determine  currents,  etc. 

Closely  allied  to  the  lateral  line  organs  in  nerve  supply  are  the 
ampullae  of  Savi  and  Lorenzini  which  occur  on  the  head  of  elasmo- 
branchs.  Each  consists  of  a  long  tube,  opening  by  a  pore  at  the  surface 
of  the  skin  and  ending  with  a  chambered  enlargement,  the  ampulla, 
at  the  deeper  end.  The  tube  is  filled  with  a  crystal  mucus  and  the 
ampulla  is  embraced  by  fibres  of  the  lateralis  nerve.  The  organs 
have  been  supposed  to  be  connected  with  a  pressure  sense.  The 
statement  is  made  that  when  they  are  removed  the  fish  is  unable  to 
sink;  this  may  throw  some  light  on  their  functions. 

The  Auditory  Organs. 

Both  in  character  of  innervation  and  in  certain  peculiarities  of 
development  the  sensory  parts  of  the  vertebrate  ears  are  closely  related 
to  the  lateral  line  organs.  In  their  most  complete  expression  three 
parts  are  recognized  in  the  auditory  organs,  the  outer,  middle  and 
inner  ears.  Of  these  the  last  is  the  essential  portion  and  occurs  in  all 
vertebrates,  the  middle  ear  first  appearing  as  such  in  the  amphibia 


AUDITORY   ORGANS. 


183 


and  the  outer  ear,  more  or  less  completely  developed,  is  found  only  in 
the  amniotes. 

The  Inner  Ear  arises  as  a  circular  area  of  thickened  ectoderm  on 
either  side  of  the  head,  between  the  seventh  and  ninth  nerves  (fig.  136). 
This  soon  becomes  cup-shaped  and  then  the  cup  closes  in  to  form  an 
auditory  vesicle  (fig.  183),  the  cavity  of  which  is  connected  wuth  the 
exterior  by  a  slender  tube,  the  endoljrmph  duct,  the  result  of  incomplete 


Fig.  183. — Diagram  of  developing  human  labyrinth  from  6  to  30  mm.  long,  after 
Streeter.  a,  ampulla;  c,  cochlear  region  and  cochlea;  au,  ampullo-utricular  region;  d, 
endolymph  duct;  e,  endolymph  region;  sc,  semicircular  canal;  se,  endolymph  sac;  s,  sac- 
culus:  u,  utriculus;  us,  utriculo-saccular  canal;  v,  vestibule. 

closure.  As  one  portion  of  the  medial  wall  of  the  vesicle  develops  an 
area  of  sensory  epithelium  like  that  of  the  lateral  line  system,  this 
stage  may  be  compared  to  an  isolated  canal  organ  with  a  single  pore. 

In  the  amphibia  and  some  of  the  ganoids,  where  there  is  a  two-layered  ectoderm 
from  the  early  stages,  there  is  never  an  open  auditory  cup.  The  lower,  so-called 
nervous  layer  of  the  ectoderm  is  alone  concerned  in  the  formation  of  the  auditory 
vesicle,  while  the  outer  layer  extends  as  an  unbroken  sheet  across  the  cup.  In 
the  elasmobranchs  the  endolymph  duct  opens  to  the  exterior  throughout  life,  the 
external  pores  being  recognizable  on  the  top  of  the  head.  Elsewhere  they  later  lose 
their  external  openings,  and  the  distal  end  of  each  usually  expands  into  an  enlarge- 
ment, the  sacculus  endolymphaticus ;  but  in  the  amphibia  the  ducts  of  the  two 
sides  may  unite  dorsal  to  the  brain,  while  other  parts  may  branch  and  grow  in  a 
root-like  manner,  in  the  canal  of  the  spinal  cord,  sending  diverticula  (frog)  into  the 
so-called  calcareous  glands,  which  surround  the  basal  parts  of  the  spinal  nerves. 

The  next  stage  in  the  auditory  vesicle  is  its  differentiation  by  a 
constriction  into  two  chambers,  an  upper  vestibulum  or  utriculus 


i84 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


and  a  lower  sacculus  (fig.  183),  the  two  connected  by  a  narrow  sac- 
culo-utricular  canal.  The  sensory  area  becomes  divided  between 
the  two,  but  the  endolymph  duct  is  connected  with  the  sacculus  alone. 
The  anterior,  posterior  and  lateral  walls  of  the  utriculus  now  produce 
flattened  outgrowths,  the  lateral  in  the  horizontal,  the  others  in  vertical 
planes,  and  parts  of  the  sensory  areas  extend  into  each.  Next,  the 
walls  of  these  diverticula  become  pinched  together  so  that  each  pocket 


Fig.  184. — Diagram  of  the  membranous  labyrinth  of  a  vertebrate,  the  sensory  areas 
dotted,  ac,  anterior  semicircular  canal;  ap,  ampullae;  ca,  cristae  acusticae  in  the  ampullae; 
de,  ductus  endolymphaticus;  he,  horizontal  (external)  canal;  /,  lagena;  ml,  mn,  ms,  mu, 
maculae  of  lagena  (neglecta,  sacculi  and  utriculi);  pc,  posterior  semicircular  canal;  s, 
sacculus;  5e,saccus  endolymphaticus;  5MC,sacculo-utricuar  canal;  u,  utriculus. 


is  converted  into  a  tube  or  canal,  open  at  either  end  into  the  utriculus, 
and  hence  approximately  semicircular  in  outline.  In  one  end  of  each 
of  these  semicircular  canals  there  is  a  patch  of  sensory  epithelium 
and  the  wall  expands  around  this  into  an  ampulla,  the  ampullae  of  the 
anterior  and  external  canals  being  side  by  side,  that  of  the  posterior 
canal  at  its  lower  end. 

In  the  lower  ichthyopsida  there  is  little  differentiation  in  the  sac- 
culus, but  in  the  higher  a  pocket,  the  lagena,  is  given  off  from  its  poster- 
ior side,  a  portion  of  the  sensory  epithelium  extending  into  it.     With  in- 


AUDITORY   ORGANS. 


i8s 


creasing  powers  of  hearing  the  lagena  becomes  greatly  elongate,  until 
in  the  mammals  it  acquires  a  peculiar  development  and  is  known  as  the 
scala  media,  the  structure  and  relations  of  which  are  described  below. 

In  the  cyclostomes  utriculus  and  sacculus  are  not  differentiated.  In  the 
myxinoids  there  is  but  a  single  semicircular  canal,  with,  however,  an  ampulla  at 
either  end.  In  the  lampreys  there  are  two  canals,  both  in  the  vertical  plane,  and 
each  with  an  ampulla  at  its  lower  end. 


Fig.  i86. 

Fig.  185.^ — Labyrinth  of  human  embryo,  30  mm.  long,  after  Streeter.  a,  ampulla;  ac, 
anterior  canal;  c,  cochlea;  cr,  cms;  de,  endolymph  canal;  nc,  cochlear  nerve;  s,  sacculus;  se, 
endolymph  sac;  u,  utriculus;  v,  vestibular  nerve. 

Fig.  186. — Section  through  one  of  the  coils  of  cochlea  of  guinea  pig,  after  Schneider. 
Bone  lined;  Is,  spiral  ligament;  r,  Reissner's  membrane;  sg,  spiral  ganglion;  snt,  st,  sv, 
scalae  media  (ductus  cochlearis),  tympani  and  vestibuli. 

These  parts  of  the  internal  ear  form  the  membranous  labyrinth. 
With  the  formation  of  canals,  lagena,  etc.,  the  sensory  epithelium 
divides  into  separate  areas  (fig.  184),  some  of  which  (maculae 
acusticae)  have  sensory  cells  with  short  hairs  or  bristles,  while  others 
(cristae  acusticae),  characteristic  of  the  ampullae,  have  cells  with  longer 
hairs.  The  membranous  labyrinth  is  filled  with  a  fluid,  the  endo- 
lymph, in  which  are  solid  particles,  the  otoliths.  These  are  usually 
microscopic  crystals  of  calcium  carbonate  which  give  the  endolymph 


1 86  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

a  milky  appearance,  but  in  the  teleosts  the  lime  is  aggregated  into  one 
or  more  '  ear  stones '  of  considerable  size. 

With  the  appearance  of  cartilage  the  membranous  labyrinth  be- 
comes enclosed  in  a  protecting  otic  capsule  (p.  60),  which  usually 
follows  pretty  closely  the  divisions  and  canals  of  the  epithelial  parts, 
thus  forming  the  skeletal  labyrinth,  separated  from  the  membranous 
labyrinth  by  a  slight  gap  filled  with  fluid  (the  perilymph).  When 
ossification  occurs  the  skeletal  labyrinth  is  converted  into  the  several 
otic  bones.  Sometimes  the  perilymph  space  is  separated  from  the 
brain  cavity  by  membrane  alone,  but  usually  firmer  structures  inter- 
vene, interrupted  only  by  foramina  for  the  passage  of  nerves  and  blood- 
vessels, for  the  endolymph  duct  and  for  a  similar  perilymph  duct 
which  extends  downward.  On  the  other  hand,  in  all  vertebrates  in 
which  the  middle  ear  is  developed  the  lateral  part  of  the  skeletal  wall 
has  two  openings  into  the  middle  ear.  The  lower  of  these  (fig.  188), 
the  fenestra  tympani  (f.  rotunda),  is  closed  by  membrane.  In  the 
upper  (fenestra  ovale  or  vestibuli)  the  membrane  supports  a  small 
bone,  the  stapes  (p.  73). 

One  part  of  this  compound  skeletal  and  membranous  labyrinth  of 
the  mammals  becomes  very  complicated.  The  lagena  becomes  greatly 
elongated  and  in  order  to  accommodate  its  length  it  is  coiled  in  a 
spiral,  its  sides  reaching  the  walls  of  the  skeletal  labyrinth  on  either 
side.  In  this  way  the  perilymph  space  is  divided  into  two  spiral  tubes 
(fig.  186),  called  scalae,  from  their  resemblance  to  spiral  stairways. 
The  upper  of  these  is  the  scala  vestibuli,  the  lower  the  scala  tympani, 
while  the  scala  media  is  formed  by  the  lagena.  This  whole  part  of 
the  inner  ear  is  the  cochlea,  so-called  from  its  resemblance  to  a  spiral 
shell. 

The  sense  organ  of  the  scala  media  is  very  specialized  and  is  known 
as  the  organ  of  Corti  (fig.  187).  In  general  it  may  be  said  that  the 
scala  diminishes  in  width  from  base  to  apex  of  the  cochlea,  and  is  accom- 
panied in  its  coils  by  a  branch  (cochlear)  of  the  acustic  nerve.  The 
sensory  structures  consist  of  hair  cells  and  Deiter's  cells,  regularly 
arranged,  and  a  series  of  pillar  cells,  inclined  to  each  other  like  the 
rafters  of  a  roof,  in  an  A-like  manner  (fig.  187).  As  the  A's  diminish 
in  width  from  base  to  apex  of  the  cochlea,  this  part  has  been  thought 
to  play  a  part  in  the  recognition  of  pitch.  There  is  also  a  cuticular 
structure,  the  membrana  tectoria,  which  extends  from  the  medial  wall  out 
over  the  hair  cells,  and  this  maybe  the  intermediate  organ  of  stimulation 


AUDITORY   ORGANS. 


187 


and  may  have  to  do  with  the  recognition  of  sound  waves  of  different 
rapidity.  It  has  recently  been  shown  that  the  membrana  tectoria  is 
connected  with  the  hairs  of  the  hair  cells.  The  fact  that  in  birds,  where 
pitch  is  certainly  recognized,  there  is  no  organ  of  Corti,  renders 
all  speculation  doubtful. 


Fig.  187. — Organ  of  Corti  of  guinea  pig,  after  Schneider,  d,  Deiter's  cells;  he,  Henson's 
cells;  ih,  inner  hair  cells;  ip,  inner  pillar  cells;  Is,  limbus  spiralis;  mt^  membrana  tectoria;  w, 
nerve  fibres;  oh,  outer  hair  cells;  op,  outer  pillar  cells;  si,  inner  sulcus;  st,  scala  tympani;  /, 
tunnel;  tn,  tunnel  nerve. 


The  Middle  Ear  or  tympanum  first  appears  in  the  anura.  It  con- 
sists of  a  cavity  (cavum  tympani)  in  front  of  and  below  the  otic 
capsule,  connected  by  a  slender  duct,  the  Eustachian  tube,  with  the 
phan-nx.  Externally  it  is  separated  from  the  outer  world  by  a  thin 
partition,  the  tympanic  membrane,  from  which  a  chain  of  bones,  the 
ossicula  auditus  (p.  73),  extends  across  the  cavity  to  the  fenestra  ovale, 
and  sen-es  to  transmit  the  sound  waves  to  the  inner  ear.  The  tympanic 
cavity  is  the  homologue  of  the  spiracular  cleft  of  the  elasmobranchs 
(see  respiration),  which  never  breaks  through.  The  tympanic  mem- 
brane, covered  externally  with  ectoderm,  on  the  inner  surface  with 
entoderm,  represents  the  imperforate  w^all  of  the  cleft,  while  the  Eusta- 
chian tube  is  the  narrowed  internal  end  of  the  spiracle.  The  chain  of 
ear  bones  has  already  been  described.  It  is  to  be  noted  that  the 
chain  consists  of  columella  and  stapes  in  anura  and  sauropsida, 
while  in  the  mammals  columella  is  replaced  by  incus  and  malleus. 
In  the  urodeles  and  gymnophiones,  where  no  tympanic  cavity  is  devel- 
oped, the  quadrate  articulates  with  the  stapes. 

The  External  Ear. — In  the  anura  and  in  many  reptiles  the  tym- 
panic membrane  is  flush  with  the  surface  of  the  head,  but  in  other  rep- 
tiles and  in  birds  it  is  at  the  bottom  of  a  canal,  the  external  auditory 
meatus,  the  simplest  expression  of  an  external  ear.     In  the  mammals 


i88 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


(whales,  sirenians  and  some  seals  are  exceptions)  an  external  conch  is 
developed  behind  the  meatus  to  collect  the  sound  waves  and  to  direct 
them  to  the  inner  parts.  In  some  birds  the  feathers  are  arranged 
around  the  meatus  so  as  to  play  the  same  part.  The  conch  is  strength- 
ened by  cartilage  and  is  moved  by  muscles  (fig.  142).  There  is  evi- 
dence which  points  to  the  conch  being  homologous  with  either  the 
operculum  of  fishes  or  with  the  first  external  gill  of  amphibians. 


Fig.  188. — Diagram  of  mammalian  ear.  a,  ampullae  of  semicircular  canals ;  an,  acustic 
nerve;  en,  cochlear  nerve;  em,  external  auditory  meatus;  eu,  Eustachian  tube;//,  fenestra 
tympani;  i,  incus;  nt,  malleus;  p,  perilymph  space  (black) ;  pd,  perilymph  duct;  ph,  pharynx; 
s,  stapes;  sc,  sacculus;  sm,  st,  sv,  scalae  media,  tympani  et  vestibuH;  sg,  spiral  ganglion;  t, 
tympanic  cavity;  tm,  tympanic  membrane;  u,  utriculus;  v,  vestibular  nerve. 

Functions. — The  vertebrate  ear  is  primarily  an  organ  of  equi- 
libration by  which  the  animal  recognizes  all  changes  of  position. 
Though  the  purposes  of  the  various  parts  are  not  accurately  known, 
the  following  conclusions  seem  warranted.  Every  movement  of  the 
head  afifects  the  endolymph  and  the  contained  otoliths,  causing  them 
to  move  (by  gravity  or  by  momentum,  or  by  both)  over  the  cristae 
acusticae  in  the  ampullae  and  thus  to  stimulate  the  sense  cells  and  nerves. 
The  position  of  the  semicircular  canals  in  approximately  the  three 
dimensions  of  space  would  seem  to  afford  a  means  for  the  recognition 
of  the  directions  and  amounts  of  the  components  of  any  motion.  The 
maculae,  and  especially  that  of  the  lagena,  are  probably  concerned  in  the 
recognition  of  sound.     In  the  fishes  the  lagena  is  poorly  developed, 


OLFACTORY   ORGANS.  ^89 

and  while  some  fishes  have  been  proved  to  hear,  others  have  given 
negative  results.  With  the  terrestrial  vertebrates  the  sound  percipient 
functions  of  the  ear  are  beyond  a  doubt,  while  they  still  retain  their 
equilibrational  use.  The  sound  waves  strike  the  tympanic  membrane, 
are  carried  across  the  middle  ear  by  the  auditory  ossicles,  and  set  the 
perilymph  in  motion  and  thus  affect  the  parts  of  the  membranous 
labyrinth. 

Organs  of  Taste. 

The  sense  of  taste  is  resident  in  groups  of  cells  known  as  taste  buds. 
These  differ  morphologically  from  the  lateral  line  organs  in  having 
each  sensory  cell  extend  the  depth  of  the  bud,  ending  at  the  basal  mem- 
brane, while  the  majority  of  the  supporting  cells  are  on  the  outer  side  of 
the  bud.  Each  sense  cell  bears  a  short,  bristle-like  percipient  struc- 
ture on  its  free  end,  while  the  basal  end  is  embraced  by  the  fibrillae  of  the 
nerve.  According  to  the  accounts  of  the  development  the  taste  buds 
are  derived  from  the  entoderm,  the  only  case  apparently  established 
for  the  origin  of  sense  organs  except  from  the  ectoderm.  In  the  higher 
vertebrates  the  organs  are  restricted  to  the  cavity  of  the  mouth  where 
(mammals)  they  occur  on  the  tongue,  especially  on  and  near  the  cir- 
cumvallate  papillae,  on  the  soft  palate  and  on  the  epiglottis.  In  the 
fishes  the  distribution  is  much  wider,  for  they  are  found  in  the  pharynx, 
on  the  gills,  and  in  many  species  on  the  surface  of  the  body,  even  upon 
the  tail.  The  barbels  about  the  mouth  of  many  forms  are  richly 
supplied  with  these  organs. 

The  taste  organs  are  supplied  by  different  nerves.  Apparently 
those  of  mammals  are  supplied  by  the  chorda  tympani  and  the  lingual 
branch  of  the  ninth  nerve.  In  the  fishes  those  of  the  pharyngeal 
region  are  supplied  by  the  post-trematic  branches  of  the  glossopharyn- 
geal and  vagus;  those  of  the  mouth  by  the  palatine  and  mandibular 
branch  of  the  seventh;  while  those  on  the  head  of  teleostomes  are 
supplied  by  the  ophthalmic  and  maxillary  branches  of  the  fifth;  and 
those  of  the  trunk  by  the  nerve  of  Weber  (p.  173),  formed  by  fibres 
from  the  seventh  and  sometimes  of  the  tenth  nerves. 

Olfactory  Organs. 

While  the  senses  of  smell  and  taste  are  closely  associated  physiolog- 
ically, being  what  might  be  called  the  chemical  senses,  the  organs  con- 


190         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

cerned  differ  considerably  in  structure  and  relations.  The  olfactory 
epithelium  is  always  restricted  to  one  or  two  patches  at  the  anterior  end 
of  the  head  and  differs  from  the  taste  buds  in  histological  structure. 
Both  sensory  and  supporting  cells  of  the  olfactory  organs  are  variously 
constituted.  The  supporting  cells  are  the  stouter,  some  being  ciliated, 
some  muciparous  at  their  free  ends.  The  sense  cells  (fig.  176,  C)  are 
thread-like  or  rod-like,  being  greatly  expanded  around  the  spherical 
nucleus,  while  the  basal  end  of  each  contracts  to  a  nerve  fibre  which 
extends  back  to  the  olfactory  tract  (p.  168),  where  the  dendrites,  inter- 
lacing with  those  of  the  olfactory  lobe,  form  the  glomeruli.  In  the 
higher  vertebrates  a  third  kind  of  cislls,  the  basal  cells,  occur  at  the  base 
of  the  olfactory  epithelium. 

The  olfactory  epithelium  arises  as  part 
of  the  surface  ectoderm  of  the  top  of  the 
head,  but  with  growth  it  changes  its  position. 
For  protection  it  sinks  beneath  the  surface  as 
an  olfactory  sac,  connected  with  the  external 
world  by  (usually)  a  pair  of  openings,  the  ex- 
ternal nares.  The  growth  of  the  dorsal  side 
of  the  head  carries  the  nares  toward  the  tip 
of  the  snout  and,  in  the  elasmobranchs,  to 
the  ventral  side  of  the  head. 

The    accessory    parts    of    the    olfactory 
„        „      ^^     ,  ,  organs    are    the    skeletal  nasal  capsules  (p. 

Fig.   189.— Nasal  organ  of  ,      °  .  r  \r 

caEcilian(£/>imMw), after  Sara-  62),  which  are  always  present;  in  the  tetra- 
t»bsfn^,trriraldu??/ft  PO^ous  forms  gknds  to  keep  the  epithelium 
lateral  cavity;  mp,  middle  pas-  moist,  and  the  organ  of  Tacobson.  The  in- 
sage;  OS,  olfactory  sac.  ,      .            p     ^                 ^ 

volution  of  the  nasal  sacs  necessitates  some 
mechanism  for  bringing  the  external  medium  (water  or  air)  to  the  sen- 
sory cells.  These  will  be  described  in  connection  with  the  several 
groups  below.  The  organ  of  Jacobson  is  a  kind  of  accessory 
olfactory  organ,  first  appearing  in  the  amphibia,  supplied  by  the  first 
and  fifth  nerves  and  apparently  serving  to  test  the  character  of  the  food 
while  in  the  mouth.  The  position  of  the  organ  near  the  internal 
nostrils  lends  probability  to  this  view  of  the  function. 

The  cyclostomes  differ  markedly  from  the  other  vertebrates  in  their  olfactory 
organs.  The  unpaired  area  of  olfactory  epithelium  develops  in  the  region  of  the 
anterior  neuropore  (p.  12)  and  becomes  involved  v^^ith  the  involution  for  the 
hypophysis  (fig.  190)  so  that  there  is  but  a  single  external  opening,  serving  for  both 


OLFACTORY   ORGANS. 


191 


olfactory  organ  and  hypophysis.  Hence  cyclostomes,  having  but  a  single  nostril, 
are  called  monorhinal,  in  comparison  with  all  other  vertebrates  which  have  two 
nostrils  (amphirhinal).  The  median  opening  or  naris  of  the  cyclostomes  connects 
with  a  naro-hypophysial  duct,  on  the  upper,  posterior  wall  of  which  is  the  ol- 
factory sac,  formed  of  pairs  of  lateral  folds  (fig.  191)  covered  with  the  olfactory 


Fig.  190. — Longitudinal  section  of  head  of  19  day  Petromyzon  embrgyo.  ch,  optic 
chiasma;  ep,  epiphysial  outgrowth;  h,  hypophysial  ingrowth;  mes,  mesenteron;  n,  nasal 
epithelium;  nc,  notochord;  oc,  oral  ca\dty;  op,  oral  plate;  sc,  canal  of  spinal  cord;  th, 
thjTcoid. 

epithelium  and  supplied  by  a  pair  of  olfactory  nerves.  The  lower  part  of  the  duct, 
now  purely  hypophysial,  descends  to  the  hypophysis  on  the  ventral  side  of  the  brain, 
where  it  either  ends  blindly  (petromyzons)  or  opens  into  the  dorsal  part  of  the  oral 
cavity  (myxinoids).  In  the  latter  group  the  olfactory  organ  is  surrounded  by  a 
complicated  nasal  capsule  of  enormous  size  (fig.  153). 


on 


Fig.  191  Fig.  192. 

Fig.   191. — Xario-hypophysial  region  oi  Petromyzon,  iromaboxe.     c,  cartilage  of  nasal 
capsule;  hd,  hypophysial  duct;  of,  folds  of  olfactory  membrane;  on,  olfactory  nerve. 
Fig.  192.— Head  of  Murcena,  after  Jordan  and  Evermann,  showing  double  nostrils. 


All  Other  vertebrates  have  paired  olfactory  areas  and  paired  nostrils 
(nares)  are  developed  in  connection  with  them,  and  they  have  at  no 
time  any  relation  to  the  hypophysis.  The  mechanism  for  bringing  the 
water  or  air  to  be  tested  to  the  olfactory  surface  differs  accordingly  as 
the  animals  are  air  or  water  breathers.  In  all  fishes,  with  the  exception 
of  the  dipnoi,  the  sensory  surface  is  at  the  bottom  of  a  pit  with  no 
connection  with   the   alimentary   canal.     In   the   elasmobranchs,   in 


192 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


order  that  water  may  flow  more  readily  through  this  pit,  a  fold  is 
developed  on  one  side  of  each  naris,  which  practically  divides  it  into 
two.  In  many  teleosts  there  is  an  actual  division  of  each  primitive 
nostril  into  two,  which  may  be  at  some  distance  from  each  other, 


Fig.  193. — Section  through  the  nasal  labyrinth  of  Polypterus.     The  nerve  runs  through 

the  centre. 


'~^i-vy 


Fig.  194.  Fig.  195. 

Fig,  194. — ^Head  of  chick  of  5^  days,  showing  development  of  oro-nasal  canal 
after  Keibel.  cf,  chorioid  fissure;  /,  thickening  for  lacrimal  duct;  n,  nasal  pit;  on,  oro- 
nasal  groove. 

Fig.  195. — Model  of  mouth  of  Echidna  embryo,  after  Seydel,  showing  method  of  in- 
growth of  palatal  folds  {pf)  to  cut  ofif  secondary  nasal  passages,  ch,  primitive  choanae;  et, 
egg  tooth;  7,  opening  of  Jacobson's  organ. 

often  at  the  ends  of  prominent  tubes  (fig.  192).  Inside  the  nasal 
capsule  the  olfactory  epithelium  is  variously  folded  in  order  to  increase 
the  sensory  surface,  often  forming  a  labyrinth  of  considerable  complex- 
ity (fig._  193). 


OLFACTORY   ORGANS. 


193 


In  air-breathing  vertebrates,  beginning  with  the  dipnoi,  a  means  is 
developed  for  drawing  air  over  the  sensory  surface,  the  first  traces  of 
which  are  seen  in  the  elasmobranchs.  These  frequently  have  an 
oro-nasal  groove,  leading  from  each  naris  to  the  angle  of  the  mouth. 
In  some  species  this  groove  is  practically  converted  into  a  tube  by  the 
meeting  of  the  walls  below.  Beginning  with  the  dipnoi  and  continuing 
with  the  amphibia  and  amniotes  (fig.  194)  a  similar  groove  is  formed 
on  either  side  before  the  formation  of  skeletal  parts.  This  closes  in, 
the  edges  of  each  groove  uniting,  so  that  a  tube  or  duct  is  formed,  lead- 
ing from  the  naris  into  the  oral  cavity,  where  an  internal  naris  or 
choana  occurs.  Later  maxillary  and  premaxillary  bones  arise  ventral 
to  the  narial  passage,  so  that  the  ducts  appear  to  run  through  the  skull. 
The  position  of  the  choanae  varies  considerably,  being  just  inside  the 
jaws  in  the  amphibia  and  lower  reptiles,  farther  back  in  the  higher 
reptiles  and  the  birds  and  mammals,  the  nasal  passages  being  cut  off 
from  the  roof  of  the  primitive  mouth  by  the  ingrowth  of  the  palatal 
processes  of  the  maxillary  bones  and  higher,  by  similar  extensions  of 
the  palatines,  and  in  some  cases,  of  the  pterygoids  (fig.  195). 

Incomplete  closure  of  the  oronasal  groove  results  in  the  deformity  known  as 
'hare-lip'  externally,  while  'cleft  palate'  is  the  result  of  failure  of  palatines,  and 
sometimes  of  maxillaries  to  meet  below  the  nasal  passages. 


Fig.  196.  Fig.  197. 

Fig.  196. — Section  through  the  nasal  region  of  Siren,  after  Seydel.  en,  nasal  cavity, 
jg\  Jacobson's  gland;  jo,  organ  of  Jacobson;  v,  vomer. 

Fig.  197. — Section  of  nose  oi  Chelonia  cauana,  aitei  Gegenbaur.  c,  concha;  ch,  choana; 
i,  inner  olfactory  groove;  n,  projection  of  naris  between  dotted  lines. 

In  the  dipnoi  the  olfactory  membrane  forms  a  few  large  folds  on 
the  dorsal  side  of  the  respiratory  duct  formed  from  the  oronasal  tube. 
In  the  amphibia  the  sensory  surface  has  a  similar  position  on  the  upper 
medial  surface  (fig.  196),  with  frequently  a  lateral  pocket  lined  with 
sensory  epithelium,  the  beginnings  of  an  organ  of  Jacobson.  In  the 
same  group  glands   (inner  and  outer  Jacobson's  glands)  occur  for 


194        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

moistening  the  olfactory  epithelium.  Usually  there  is  little  complica- 
tion of  the  olfactory  surface,  but  in  a  few  urodeles  (Plethodon)  there 
is  a  projection  from  the  lateral  wall,  the  first  indication  of  the  conchae 
which  acquire  such  development  in  the  higher  groups.  There  is  fre- 
quently a  differentiation  of  the  nasal  passage  into  a  ventral  respiratory 
duct  lined  with  ordinary  and  a  more  dorsal  olfactory  duct  lined  with 
sensory  epithelium.  In  the  lower  urodeles  the  diverticulum  repre- 
senting the  organ  of  Jacobson  is  on  the  medial  side  of  the  nasal  cavity; 
a  little  higher  it  is  ventral,  while  in  the  highest  urodeles  it  has  rotated 
to  the  lateral  side.  It  may  be  noted  that  some  of  the  amphibia  have 
smooth  muscles  to  close  the  external  nares. 

Aside  from  the  varying  position  of  the  choanae  the  changes  from 
amphibia  to  reptiles  in  the  olfactory  organs  are  comparatively  slight. 


Fig.  198. — Longitudinal  section  of  nasal  region  of  alligator,  after  Gegenbaur.     c,  concha; 
ms,  maxillary  sinus;  «,  naris;  p,  pseudoconcha. 

The  olfactory  region  becomes  more  distinct  from  the  respiratory  tract 
and  the  latter  shows  a  tendency  to  be  differentiated  into  an  anterior 
atrium  or  vestibule,  a  middle  area  connected  with  the  olfactory  region, 
and  a  posterior  naso-pharyngeal  duct  between  the  basis  cranii  and 
the  roof  of  the  mouth.  This  latter  duct  varies  in  length  accordingly 
as  the  choanae  are  anterior  or  posterior  in  position,  the  extreme  being 
reached  in  the  crocodiles,  where  by  ingrowth  of  palatines  and  ptery- 
goids, the  internal  nares  are  carried  back  nearly  to  the  hinder  end  of 
the  skull.  A  single  concha,  supported  by  bone,  is  developed  in  the 
lateral  wall  of  the  reptilian  nose.  It  is  weak  in  the  turtles  (fig.  197), 
but  is  larger  elsewhere,  and  in  the  crocodiles  (fig.  198)  it  becomes 
divided  in  front,  while  a  'pseudoconch'  (its  homology  with  the  supe- 
rior concha  of  birds  is  uncertain)  is  developed  above  and  behind  the 
true  concha.  Jacobson's  organ  occurs  only  in  the  squamata,  where 
it  forms  a  simple  pocket  in  the  primitive  position,  ventral  and  medial 
to  the  nasal  cavity,  near  the  nasal  septum. 


OLFACTORY   ORGANS. 


195 


There  are  three  folds  developed  on  the  wall  of  each  nasal  cavity  in 
birds,  an  anterior  and  inferior  concha  vestibuli,  a  middle  and  a  superior 
fold,  the  middle  supported  by  the  maxillo-turbinal,  the  superior  by  the 
naso-turbinal  bones.  The  vestibular  conch  lacks  olfactory  epithelium 
at  all  times,  while  it  disappears  from  the  middle  one  after  hatching, 


Fig.  199. — Olfactory  region  of  hen  in  longitudinal  and  transverse  section,  after  Gegen- 
baur.  c,  middle  concha;  ch,  choana;  i,  inferior  (anterior)  concha;  o,  connection  of  air 
cavity  of  head;  p,  septum  of  nose;  s,  superior  concha. 

leaving  the  upper  conch  as  the  sole  seat  of  smell  in  the  adult,  which 
corresponds  with  the  limited  sense  of  smell  in  these  animals.  Jacob- 
son's  organ  is  never  developed  in  the  adult,  though  traces  of  it  appear 
in  the  embryos. 

With  the  great  increase  of  the  sense  of  smell  in  the  mammals  the 


Fig.  200.  Fig.  201. 

Fig.  200. — Model  of  the  nasal  cavity  of  a  rabbit  embryo,  13^  mm.  head  length,  after 
Peter,  ch,  choana;  el,  first  ethmoturbinal;  j,  organ  of  Jacobson;  oj,  opening  of  same;  mt, 
maxilloturbinal;  rU,  nasoturbinal. 

Fig.  201. — Nasal  cavity  of  Erinaceus,  after  Paulli,  showing  the  foldings  of  the  maxUlo- 
turbinals  (mt)  and  the  nasoturbinals  (n/). 

nasal  labyrinth  undergoes  a  corresponding  complication,  and  is  farther 
characterized  by  the  great  length  of  the  naso-pharyngeal  duct,  and  by 
the  position  of  the  olfactory  area  below  a  part  of  the  brain  cavity.  The 
folds  of  the  labyrinth  may  be  supported  by  processes,  more  or  less  com- 
plicated, of  three  bones  or  cartilages,  the  ethmo-turbinals,  the  naso- 


196 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


turbinals  and  the  maxillo-turbinals  (fig.  200),  the  purpose  of  these 
folds  being  to  increase  the  amount  of  sensory  surface,  while  the  skeletal 
supports  keep  the  folds  from  touching  each  other.  With  diminution 
of  the  powers  of  smell  the  folds  are  correspondingly  reduced,  even  to 
a  loss  of  the  turbination  of  the  bones  concerned. 

The  maxillo-turbinals  and  naso-turbinals  arise  from  the  lateral  wall 
of  the  nasal  cavity  (the  former  as  a  distinct  turbinal  bone) ,  the  ethmo- 
turbinals  as  outgrowths  from  the  ethmoid  bone, 
appearing  first  at  the  upper  hinder  part  of  the 
septal  wall  and  extending  to  the  lateral  wall. 
The  result  is  that  the  ethmo-turbinal  tends  to 
insinuate  itself  between  the  hinder  ends  of  the 
other  two  (figs.  200,  201).  Each  of  these  may  be 
subdivided,  with  corresponding  subdivision  of 
the  epithelial  covering,  and  in  the  case  of  the 
ethmo-turbinals  the  subdivisions  may  be  of 
varying  heights  (fig.  202),  the  ecto-  and  ento- 
turbinals.  The  nasoturbinals  often  disappear  in 
the  adult,  while  the  epithelium  of  the  maxillo- 
turbinals  is  not  sensory  in  character,  this  part 
of  the  nose  being  apparently  to  warm  and 
moisten  the  air  in  its  passage  to  the  lungs. 
The  homologies  of  the  various  parts  of  the  nasal  labyrinth  in  dif- 
ferent amniotes  are  thus  stated  (Peter). 

I.  Concha  of  the  anterior  epthelium:  concha  vestibuli  (birds). 

II.  Conchge  of  the  primitive  sensory  epithelium : 

1.  Arising   from   the   lateral   wall    (conchge   laterales). 

A.  Anterior: 

a.  Primary,    ventral:    concha    of    reptiles;    middle    concha 
of  birds;  maxillo-turbinals  of  mammals. 

b.  Secondary,   dorsal:  Upper  or  posterior  of  birds;  naso- 
turbinals of  mammals  (?  pseudoconch  of  crocodiles). 

B.  Arising  from  the  posterior  part:  conchae  obtectae  of  mam- 
mals. 

2.  Arising  from  the  primitively  median  wall:   ethmo-turbinals 

of  mammals,  numbered  from  in  front  backward. 
Jacobson's  organ  (vomero-nasal  organ)  is  laid  down  in  the  embryo 
of  most  mammals  as  a  groove  or  pocket  on  the  lower  medial  side  of 
each  nasal  cavity,  opening  in  rodents  and  in  man  near  the  duct  of 


Fig.  202 . — S e c t i o n 
through  the  nasal  cavity 
ofj'a  new  bom  dog,  after 
Paulli,  I-IV,  entoturbi- 
nals;  1-5,  first  to  fifth  ec- 
to turbinals. 


OLFACTORY   ORGANS. 


197 


Stenson's  gland;  in  other  mammals,  so  far  as  known,  its  duct  becomes 
cut  off  from  the  nasal  cavity  and  opens  into  the  naso-palatal  canal.  Its 
medial  wall  is  covered  with  sensory  epithelium,  supplied  by  a  branch 
of  the  olfactory  nen^e.  In  the  primates  the  organ  is  more  or  less  de- 
generate in  the  adult. 

There  are  two  kinds  of  glands  in  the  nasal  cavity,  the  smaller  and 
scattered  Bowman's  glands  and  the  larger  Stenson's  gland  lying  in 
the  lateral  ventral  wall  and  opening  into  the  vestibule.  There  are 
usually  several  sinuses  in  the  bones  of  the  skull,  connected  with  the 


Fig,  203, — Lateral  wall  of  nasal  cavity  of  man,  after  Coming,  c^,  crista  gaUi;  «',  cm, 
cs,  inferior,  middle  and  superior  conchae; fpm,  foramen  palatinum  ma.jus;fsp,  sphenopala- 
tine foramen;  ic,  incisive  canal;  osm,  opening  of  maxillary  sinus;  sf,  frontal  sinus;  ss^ 
sphenoidal  sinus. 


nasal  cavities  by  foramina.     Chief  of  these  are  the  maxillary  sinuses 
(antra  of  Highmore),  the  frontal  and  sphenoidal  sinuses  in  the 

corresponding  bones,  the  relations  of  which  may  be  seen  in  fig.  203. 
Others  may  occur  in  other  bones  of  the  face. 

Mammals  are  characterized  by  an  external  fleshy  nose,  supported 
by  the  nasal  bones  and  by  cartilages,  developed  in  part  from  the  eth- 
moid cartilage  of  the  embryo,  in  part  from  paired  cartilages,  a  new 
acquisition  of  the  mammals.  Beyond  these  skeletal  parts  is  the  fleshy 
portion  which  may  form  a  proboscis  of  considerable  size  (swine, 
elephant  shrew,  elephant). 


198 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  most  mammals  the  sense  of  smell  is  well  developed,  but  is  comparatively 
slight  in  the  seals,  whalebone  whales  and  in  the  primates,  while  it  is  completely  lost 
in  most  of  the  toothed  whales  where  even  the  olfactory  nerve  may  disappear. 


The  Eyes. 

The  sensory  part  of  the  eyes  comes  from  the  ectoderm  of  the  neural 
plate,  and  in  several  embryos  the  regions  which  are  thus  destined  may 

be  recognized  on  its  dorsal  surface 
before  it  is  infolded  to  form  the  vesi- 
cles of  the  brain.  The  accessory  parts 
of  the  eye  are  derived  in  part  from  the 
general  ectoderm,  in  part  from  meso- 
derm of  both  kinds. 

As  the  neural  plate  closes  up  to  form 
the  brain  (p.  11),  the  optic  areas  begin 
to  grow  outward  from  the  fore-brain 
toward  the  sides  of  the  head,  each  form- 
ing at  first  a  hollow  outgrowth,  the  optic 
vesicle,  connected  with  the  brain  by  a 
hollow  optic  stalk.  The  next  phase  is 
the  involution  or  invagination  of  the 
distal  side  of  the  vesicle  so  that  it  is 
converted  into  a  double  walled  optic 
cup  (fig.  204).  There  thus  results  a  differentiation  of  parts  in  the 
optic  outgrowth  and  a  partial  obliteration  of  the  cavity  of  the  vesicle. 
The  distal  wall  which  forms  the  inside  of  the  cup  is  called  the  retinal 


Fig.  204. — Stereogram  of  de- 
veloping eye.  c/",  chorioid  fissure  ;/6, 
cut  wall  of  fore-brain;  /,  anlage  of 
lens;  oc,  optic  cup;  os,  optic  stalk;  p, 
layer  for  pigmented  epithelium;  r, 
retinal  layer. 


Fig.  205. — Sections  of  successive  stages  in  the  development  of  the  lens  of  the  eye  from  the 
first  thickening  of  the  ectoderm  {ec)  to  the  complete  separation  ot  the  lens,  l. 

layer;  the  outer  wall  the  pigment  layer,  in  anticipation  of  their  de- 
velopment into  the  corresponding  parts  of  the  adult. 

The  involution  of  the  retina  is  not  easily  described,  but  may  be 


EYES. 


199 


understood  from  figure  204.  It  occurs  on  the  lower  distal  side  so  that 
the  cup  is  not  complete  but  is  interrupted  by  a  deep  notch,  the  chorioid 
fissure,  below,  and  this  is  extended  as  a  groove  on  the  ventral  side  of 
the  optic  stalk.  Later  the  fissure  closes  (fig.  194),  but  not  until 
some  of  the  changes  described  below  have  occurred. 

Opposite  the  distal  part  of  each  optic  vesicle  the  ectoderm  of  the 
side  of  the  head  thickens,  then  becomes  invaginated  (fig.  205),  the 
mouth  of  the  invagination  closes,  and  the  hollow  ball  thus  formed  is 
cut  off  from  the  rest  of  the  ectoderm  and  sinks  into  the  mouth  of  the 
optic  cup,  where  it  forms  the  lens  of  the  eye.  From  the  first  the  cells 
of  the  two  sides  of  the  lens  differ  in  size,  those  of  the  outer  wall  being 
cubical,  those  of  the  other  being  elongate,  while  the  cavity  is  a  narrow 
cleft.  Later  the  cavity  is  obliterated,  while  the  lens  is  increased  in 
size  by  the  addition  of  new  cells,  like  the  coats  of  an  onion,  by  budding 
from  the  equatorial  zone  of  the  lens. 


Fig.  206. — Mammalian  retina;  above  the  general  appearance,  below  the  diagrammatic 
relations;  the  lens  toward  the  left,  c,  cone;  cc,  cone  cell;  g,  ganglion  cells;  ig,  inner  granular 
layer;  im,  inner  molecular  layer;  m,  basal  membrane;  «/",  nerve  fibres;  og,  outer  granular 
layer;  om,  outer  molecular  layer;  r,  rod;  re,  rod  cell. 

The  Retina  consists  of  several  layers  which  constitute  the  ganglion 
and  the  sensory  cells,  the  latter  being  on  the  outer  surface,  i.e.,  that 
which  is  turned  away  from  the  lens.  Each  sensory  cell  bears  on  its 
outer  end  the  percipient  structure,  rod  or  cone,  which  has  given  these 
the  name  of  rod  and  cone  cells.  These  rods  and  cones  project 
through  the  basal  membrane  which  encloses  the  retina  into  the 
pigment  layer  to  be  described  shortly.  The  bodies  of  the  cells  with 
their  nuclei  are  inside  the  basal  membrane,  where  they  form  the  so- 


200        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

called  outer  granular  (nuclear)  layer,  separated  by  an  outer 
'molecular'  (reticular)  layer  of  interlacing  dendrites  from  the 
inner  granular  layer.  This  is  ganglionic  in  character  and  is  con- 
nected by  the  inner  molecular  layer  with  the  rest  of  the  ganglionic  layer 
which  lines  the  inside  of  the  retinal  cup. 

In  order  to  understand  the  latter  layer  and  the  relations  of  the  optic 
nerve,  an  account  of  the  development  is  necessary.  At  first  the  retinal 
layer  is  comparatively  thin,  but  it  increases  in  thickness,  in  part  by  a 
multiplication  of  cells,  in  part  by  their  increase  in  length  and  the  devel- 
opment of  the  dendrites  of  the  molecular  layers.  Each  cell  of  the  inner 
layer  (the  one  turned  toward  the  lens)  also  develops  an  axon  which 
runs  over  the  free  surface  of  the  cells  to  the  chorioid  fissure,  passes 
through  this  and  along  the  ventral  groove  of  the  optic  stalk  to  the 
diencephalon. 

As  will  readily  be  understood,  it  is  these  fibres  and  not  the  optic 
stalk  which  form  the  optic  nerve  (p.  169).  When  the  chorioid  fissure 
closes,  the  nerve  appears  to  leave  through  the  centre  of  the  retina, 
and  as  this  part  contains  no  sense  cells,  the  point  of  exit  constitutes 
the  'blind  spot'  of  physiological  works.  Besides  the  cells  already 
mentioned  the  retina  contains  supporting  or  radial  cells,  like  other 
sense  organs  or  like  the  brain  itself  (neuroglia).  These  extend  through 
from  the  nerve  fibres  to  the  basal  membrane.  Either  rods  or  cones 
may  be  absent  in  isolated  groups  of  vertebrates.  Usually  there  is  a 
spot,  the  macula  lutea  (yellow  spot)  or  fovea  centralis  at  the  centre 
of  the  retina  where  vision  is  most  distinct.  Here  the  rod  and  cone  cells 
are  shorter  and  more  crowded  than  elsewhere. 

Here  may  be  mentioned  a  point  of  morphological  importance.  It  will  be 
recalled  (p.  138)  that  the  ependymal  surface  of  the  brain  corresponds  to  the  external 
surface  of  the  ectoderm  of  the  rest  of  the  body.  Therefore,  as  a  glance  at  fig.  204 
will  show,  the  rods  and  cones  are  on  the  primitively  outer  and  the  ganglion  cells  and 
nerve  fibres  are  on  the  deeper  surface  of  the  ectoderm.  Hence  rods  and  cones 
correspond  to  the  percipient  cuticular  structures  of  other  sensory  organs  like  the 
lateral  line,  taste  buds  and  the  like.  Before  it  can  affect  the  sensory  cells  the  light 
has  to  traverse  the  whole  of  the  retina  and  then  the  nervous  impulses  have  to 
pass  back  through  the  same  layers  to  reach  the  optic  nerve.  This  constitutes  an 
'inverted  eye'  and,  with  the  exception  of  a  few  molluscs,  it  is  unknown,  except  in 
the  vertebrates.  A  comparison  with  the  parietal  eye  of  reptiles  (fig.  151)  is  very 
instructive. 

The  cavity  between  lens  and  retina  is  filled  with  a  semisolid  vitre- 
ous body,  the  origin  of  which  is  in  dispute.     In  mammals  blood-vessels 


EYES. 


20I 


and  mesenchymatous  cells  enter  the  optic  cup  through  the  chorioid 
fissure  before  its  closure.  Some  suppose  that  the  vitreous  body  arises 
from  a  modification  of  these  cells,  some  regard  it  as  an  exudate  from  the 
blood-vessels,  and  others  think  it  a  retinal  secretion.  The  fact  that 
the  blood-vessels  mentioned  do  not  occur  in  birds  is  of  interest  in  this 
connection.  In  mammals,  when  the  chorioid  fissure  closes,  the  vessels 
appear  to  enter  through  the  centre  of  the  optic  nerve  (central  retinal 
artery  and  vein — fig.  207).     In  the  early  stages  the  retinal  artery 


Fig.  207. — Diagrammatic  section  of  half  a  mammalian  eye.  ac,  anterior  chamber; 
ca,  ciliary  arteries;  c,  eyelash  (cilium);  cj,  conjunctiva;  co,  cornea;  cp,  ciliary  process;  cr, 
central  retinal  artery  and  vein;  cs,  conjunctival  sac;  ct,  chorioid  tunic;  d,  dura  of  optic 
nerve;  i,  iris;  on,  optic  nerve;  os,  oia.  serrata;  pc,  posterior  chamber;  pe,  pigmented  epithel- 
ium; r,  retina;  sc,  sclera;  tg,  tarsal  gland;  w,  vorticose  vein;  cc,  zonula  zinii. 

divides  inside  the  cup,  one  branch  (hyaloid  artery)  going  through  the 
vitreous  body  to  the  neighborhood  of  the  lens,  the  other  being  distrib- 
uted over  the  inner  surface  of  the  retina.  Later  the  hyaloid  artery 
disappears,  while  retinal  arteries  are  rare  except  in  mammals. 

The  outer  wall  of  the  optic  cup  forms  the  pigmented  epithelium 
of  the  eye,  developing  a  large  amount  of  black  pigment  which  eventu- 
ally surrounds  and  isolates  the  rods  and  cones,  so  that  each  can  be 
affected  only  by  the  light  which  falls  directly  upon  it.  As  will  readily  be 
understood  the  side  of  the  pigment  layer  away  from  the  retina  corre- 
sponds to  the  deeper  surface  of  the  skin  and  so  comes  into  relation  with 
the  connective  tissue.  From  this  is  developed  the  envelopes  of  the 
eye — tunica  vasculosa,  sclera,  etc. 

Surrounding  the  retina  and  pigmented  epithelium  and  extending 
forward  over  the  lateral  parts  of  the  lens  is  the  tunica  vasculosa, 
in  which  two  parts  are  recognized,  the  iris  and  the  chorioid.     The 


202        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

whole  is  richly  vascular,  and  the  chorioid,  supplied  by  the  ciliary 
arteries  which  enter  at  the  side,  is  the  chief  source  of  nourishment 
for  the  rod  and  cone  cells.  To  the  vascular  part  certain  other  portions 
are  added  in  some  groups.  Thus  just  outside  the  blood-vessels  there 
may  be  a  large  lymph  space,  and  outside  of  this,  in  most  fishes  and 
some  amphibia  and  turtles,  there  is  an  argenteal  layer  containing 
calcic  crystals  which  give  the  layer  a  whitish  appearance.  On  the 
other  hand,  the  side  toward  the  retina  frequently  develops  a  somewhat 
similar  tapetum  lucidum,  with  a  metallic  lustre,  which  reflects 
light  strongly  and  is  the  cause  of  the  apparent  shining  at  night  of  the 
eyes  of  many  selachians  and  some  other  fishes  and  carnivore  mammals. 
In  a  few  teleostomes  (those  with  a  pseudobranch)  there  is  a  so-called 
chorioid  gland  just  oustide  the  vascular  layer,  near  the  entrance  of 
the  optic  nerve.     It  partakes  of  the  nature  of  a  rete  mirabile. 

The  chorioid  extends  as  far  forward  as  does  the  retina,  when  its 
anterior  edge  is  produced  into  a  circular  ciliary  process,  which  is  best 
developed  in  the  amniotes,  though  appearing  here  and  there  in  the 
ichthyopsida.  This  process  is  muscular  (ciliary  muscles)  at  its  base  and 
is  connected  at  its  margin  with  the  delicate  capsule  surrounding  the 
lens  by  a  double  fenestrated  membrane,  the  zonula  ciliaris  (Zinnii). 
By  the  action  of  the  muscles  the  lens  is  moved  toward  or  away  from  the 
retina,  while  variations  in  tension  may  slightly  alter  its  shape,  thus 
changing  its  focal  point  (accommodation  of  the  eye). 

Beyond  the  ciliary  process  the  vascular  tunic  continues  in  front 
of  the  lens  as  the  iris,  a  circular  curtain  with  a  central  opening,  the 
pupil.  Pigment  in  the  posterior  layer  (uvea)  of  the  iris  renders  it 
opaque,  while  in  many  fishes  the  outer  surface  is  silvery  owing  to  the 
continuation  of  the  argentea  into  this  region.  The  rest  of  the  iris  is 
muscular,  the  muscles  increasing  in  extent  from  the  lower  to  the 
higher  forms.  They  are  arranged  in  two  groups.  The  circular  muscles 
(sphincter  pupillae),  by  their  contraction,  diminish  the  size  of  the  pupil; 
the  radial  (dilator  pupillae)  are  antagonistic  and  efifect  an  enlarge- 
ment of  the  opening  in  the  iris.  In  the  sauropsida  these  iridial  muscles 
are  cross  banded,  in  amphibia  and  mammals  of  the  smooth  variety. 

Surrounding  all  of  the  structures  of  the  eye  so  far  described  is  the 
sense  capsule,  which  differs  from  all  other  sense  capsules  (p.  62)  in 
not  being  connected  with  the  rest  of  the  skull,  as  a  result  of  its  neces- 
sity for  movement.  In  the  capsule  two  parts  are  distinguished,  the 
sclera  which  covers  the  proximal  side  of  the  eye,  and  the  cornea, 


EYES.  203 

perfectly  transparent,  through  which  light  passes  to  reach  the  lens. 
The  cornea,  covered  externally  by  the  conjunctiva,  the  modified 
epidermis  of  the  front  of  the  eye,  consists  of  connective  tissue;  the 
sclera  is  usually  white.  In  most  of  the  lower  vertebrates  and  in  the  mono- 
tremes  it  is  partly  or  wholly  cartilaginous,  but  in  other  mammls  and  in 
the  lampreys  it  consists  of  fibrous  tissue.  In  the  stegocephals  and  in 
many  reptiles  and  birds  portions  of  the  sclera  ossify  as  a  ring  of  scle- 
rotic bones  (p.  67). 

Sclerotic  bones  are  lacking  in  snakes,  plesiosaurs  and  crocodiles.  In  the 
sturgeon  and  many  teleosts  two  or  more  dermal  bones  develop  upon  the  sclera,  but 
neither  these  nor  the  calcifications  to  be  found  in  some  sharks  and  teleosts  are  to 
be  confused  wdth  true  sclerotics. 

Between  cornea  and  lens  is  a  cavity  which  is  partially  divided  by 
the  iris  into  anterior  and  posterior  chambers  which  connect  with 
each  other  through  the  pupil.  These  are  filled  with  a  refracting  fluid, 
the  aqueous  hiunor. 

The  parts  so  far  described  form  the  eye-ball  (bulbus  oculi)  which 
is  more  or  less  freely  movable  in  its  socket  in  the  side  of  the  head.  It 
is  moved  by  the  six  muscles  (p.  128)  which  are  constantly  present. 
Others  may  occur  here  and  there.  Thus  in  the  amphibia  a  distinct 
muscle  (retractor  bulbi)  is  developed  from  the  external  rectus  to 
pull  the  ball  back  into  the  socket,  while  portions  of  the  jaw  muscles 
may  be  set  apart  as  elevators  and  depressors  of  the  ball.  In  the  elasmo- 
branchs  a  cartilaginous  rod,  the  optic  pedicel,  extends  from  the  ball 
to  the  skull.  This  is  replaced  in  the  teleosts  by  a  fibrous  band,  the 
tenaculum,  but  its  equivalent  is  not  found  in  the  higher  groups. 

Among  the  accessory  parts  of  the  eye  are  the  lids,  of  which  there 
may  be  three,  the  upper  and  the  lower  lids  so  familiar  in  the  higher 
vertebrates  and  the  third  lid,  the  nictitating  membrane,  a  transparent 
sheet  which  may  be  drawn  horizontally  across  the  front  of  the  eye 
from  the  inner  (anterior)  angle  of  the  eye  or  from  beneath  the  lower  lid. 
All  three  lids  are  folds  of  the  skin.  The  upper  and  lower  are  poorly 
developed  in  the  ichthyopsida,  but  appear  in  the  amniotes.  They 
are  lined  on  the  side  next  the  eye  by  a  continuation  of  the  conjunctiva, 
which  continues  beyond  the  edge  of  the  lid  as  the  epidermis.  The 
nictitating  membrane  appears  in  some  sharks,  again  in  the  amphibia, 
and  receives  its  highest  development  in  the  sauropsida,  while  in  the 
mammals  it  is  reduced  to  a  rudimentary  fold,  the  plica  semilunaris, 
at  the  inner  angle  of  the  eye. 


204  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

There  are  no  glands  connected  with  the  eyes  in  cyclostomes  or 
fishes,  but  in  the  urodeles  a  series  of  glands  is  developed  from  the  con- 
junctival lining  of  the  lower  lid.  In  the  amphibia  they  show  little 
differentiation,  but  in  all  sauropsida  (glands  are  lacking  from  a  few 
reptiles — crocodile  tears  are  non-existent)  they  become  divided  into 
two  groups.  One  becomes  aggregated  near  the  inner  angle  and  forms 
what  is  known  as  Harder*s  glands  (glandula  membrana  nictitans) ; 
the  others  migrate  toward  the  outer  angle  of  the  eye  and  constitute  the 
true  lacrimal  or  tear  gland.  In  the  mammals  the  migration  continues 
until  the  gland  comes  to  lie  beneath  the  upper  lid,  where  it  shows  its 
multiple  nature  by  the  numbers  of  ducts  by  which  it  pours  its  secretions 
into  the  conjunctival  sac.  In  most  mammals  Harder's  gland  degener- 
ates. The  tears  secreted  by  the  glands  pass  over  the  conjunctiva  and  are 
collected  at  the  inner  angle  of  the  eye,  where  they  are  drained  by  the 
lacrimal  duct  into  the  cavity  of  the  nose.  This  duct  is  formed  as  a 
thickening  of  the  epidermis  which  later  becomes  perforated.  It 
follows  the  course  of  an  earlier  groove  (fig.  194)  leading  from  the  orbit 
to  the  nasal  invagination  and  which  was  formerly  thought  to  form  the 
duct. 

The  eyes  of  the  cyclostomes  are  degenerate.  In  the  larval  (Ammocoetes)  stage 
of  Petromyzon  the  eye  is  buried  under  a  thick  skin,  but  this  thins  out  in  the  adult. 
In  the  myxinoids  the  lens  and  eye  muscles  are  lacking,  and  iris,  cornea  and  sclera 
are  not  dififerentiated. 

Fishes  have  eyes  with  a  very  flattened  cornea,  a  spherical  lens  and  very  long 
retinal  rods.  A  peculiar  feature  in  many  fishes  is  the  falciform  process,  a  vascular 
and  muscular  structure  which  enters  the  retinal  cup  through  the  chorioid  fissure  and 
extends  to  the  lens  where  it  bears  an  expansion,  the  campanula  Halleri.  The 
whole  is  supposed  to  act  as  a  means  of  accommodation,  there  being  no  ciliary 
muscles.  In  most  fishes  the  eyes  are  so  placed  on  the  sides  of  the  head  that  there 
must  be  monocular  vision.  In  the  flat  fishes  (Heterosomata)  one  of  the  eyes  mi- 
grates during  development,  so  that  both  eyes  come  to  lie  on  one  side  of  the  head. 

Most  sauropsida  are  characterized  by  the  development  of  a  process  from  the 
inner  retinal  surface  which  reaches  its  extreme  in  the  pecten  of  the  birds.  In  the 
reptiles  it  is  a  small  conical  process  arising  from  the  point  of  entrance  of  the  optic 
nerve,  but  in  the  birds  this  expands  distally  into  a  quadrangular  plate,  folded  like  a 
fan,  to  which  various  functions  have  been  ascribed.  It  has  been  recently  shown 
to  be  rich  in  sense  cells.  The  shape  of  the  eye  of  the  bird  is  peculiar,  but  is  not 
easily  described.  It  consists  of  a  hemispherical  posterior  part,  followed  by  a 
conical  portion,  and  this  surmounted  by  a  hemispherical  corneal  region,  the  whole 
being  somewhat  telescopic  in  shape.  The  whole  is  very  large  in  proportion  to  the 
size  of  the  animal. 

The  pecten  is  said  to  be  outlined  in  the  foetal  stages  of  some  mammals.     The 


DIGESTIVE   ORGANS.  205 

pupil  of  the  mammals  is  not  always  circular,  but  is  a  vertical  slit  in  the  cats,  a 
horizontal  opening  in  the  whales,  many  ungulates,  etc.  During  development  the 
lids  fuse  for  a  time,  separating  in  some  cases,  only  after  birth.  The  edges  of  the 
lids  are  fringed  with  short  hairs,  the  eye-lashes  or  cilia,  and  internal  to  these  are 
the  ducts  of  sebaceous  glands  (tarsal  or  Meiobomian  glands),  the  glands  them- 
selves being  in  the  substance  of  the  lids.  The  whales  have  an  enormously  thick 
sclera  which,  here  as  elsewhere,  appears  as  a  continuation  of  the  dural  sheath  of 
the  optic  nerve. 


THE  DIGESTIVE  ORGANS. 

Few  articles  of  food,  as  they  come  to  a  vertebrate,  are  in  shape  to  be 
taken  immediately  into  the  organism  and  to  be  used,  without  modifica- 
tion, as  a  source  of  energy  or  as  material  for  the  construction  of  new 
tissue  or  the  repair  of  old.  They  have  to  be  altered  so  that  they  are 
soluble  and  so  able  to  pass  by  osmosis  into  the  blood-vessels  (proteids, 
carbohydrates),  or  they  must  be  broken  up  (hydrocarbons)  so  as  to  be 
taken  up  by  the  absorbtive  vessels  (lacteals)  of  the  lymphatic  system. 
These  changes  in  the  food,  which  are  the  result  of  the  action  of  the 
secretions  of  the  digestive  glands,  constitute  the  process  of  digestion. 
The  digestive  tract  or  alimentary  canal,  where  these  changes  take 
place,  also  has  to  provide  for  the  passage  of  the  digested  food  into  the 
blood-vessels,  to  be  carried  by  them  to  all  parts  of  the  body.  It  is  there- 
fore richly  supplied  with  blood-  and  lymph-vessels. 

The  alimentary  canal,  which  is  complete  (i.e.,  has  both  mouth  and 
vent),  is  largely  entodermal  in  origin,  but  small  portions  at  either  end 
are  derived  from  the  ectoderm.  The  entodermal  portion,  the  mesen- 
teron,  consists  of  the  wall  of  the  archenteron  (p.  12)  after  the  separa- 
tion of  the  notochord,  the  mesothelium,  and  a  few  less  prominent  struc- 
tures. The  ectodermal  parts  are  a  stomodeum  at  the  cephalic  end  and 
a  proctodeum  behind. 

In  the  early  stages  of  all  vertebrates  the  mouth  is  lacking,  the  ce- 
phalic end  of  the  archenteron  abutting  directly  against  the  ectoderm  of 
the  ventral  side  of  the  head,  so  that  an  oral  (pharyngeal)  plate  is 
formed,  consisting  of  both  ectoderm  and  entoderm.  Next  this  plate  is 
pushed  inward,  either  as  a  pocket  (fig.  190)  or  as  a  solid  plug,  carrying 
the  entoderm  before  it.  This  ingrowth  constitutes  the  stomodemn, 
and  the  site  of  its  ingrowth  forms  the  mouth  opening  of  the  adult. 
Later  the  oral  plate  breaks  through,  placing  the  stomodeum  and 
mesenteron  in  communication. 


2o6 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  the  majority  of  vertebrates  the  blastopore  closes  behind,  so  that 
the  anus  is  a  new  formation,  although  it  arises 
in  the  line  of  closure.  In  the  amniotes  this 
opening  is  preceded  by  the  formation  of  a 
pocket,  the  proctodeum,  similar  to  the 
stomodeum,  and  opening  later  into  the 
mesenteron  in  the  same  way.  In  the  adult  it 
is  impossible  to  find  any  lines  separating  the 
three  regions,  stomodeum,  mesenteron  and 
proctodeum. 

The  proctodeum  lies  wholly  behind  the  entrance 
of  the  urogenital  ducts  into  the  cloaca.  The  ecto- 
derm of  the  stomodeum  extends  inward  as  far  as  the 
posterior  teeth,  following  the  outline  of  the  jaws.  On 
the  dorsal  side  of  the  oral  cavity  two  pits  persist  for 
some  time,  the  limits  of  ectoderm  and  entoderm  pass- 
ing between  them.  The  posterior  of  these,  SeessePs 
pocket,  is  of  unknown  significance.  The  other, 
Rathke's  pocket  (fig.  253),  lies  just  in  front  of  the 
oral  plate.  It  marks  the  point  of  invagination  of  the 
hypophysis  (p.  148)  and  remains  open  for  a  time  as 
the  hypophysial  duct  (fig.  148). 

In  some  teleosts,  where  the  stomodeal  ingrowth  is 
slight,  the  mouth  appears  at  first  as  a  pair  of  per- 
forations in  the  oral  plate,  these  later  coalescing  to 
form  the  permanent  mouth.  This  condition  lends 
plausibility  to  the  view  that  the  vertebrate  mouth  has 
arisen  from  the  coalescence  of  a  pair  of  gill  clefts. 

Except  in  the  higher  mammals  the  ento- 
dermal  part  of  the  alimentary  canal  contains  a 
large  amount  of  food  yolk  in  the  early  stages. 
In  the  sauropsida  this  is  so  abundant  that  the 
whole  cannot  be  contained  in  the  body  walls, 
and  hence  it  causes  the  ventral  side  of  the 
canal  to  protrude  as  a  yolk-sac,  which  is 
gradually  absorbed  with  the  digestion  and  re- 
moval of  the  yolk  by  the  blood-vessels. 

The  first  differentiation  in  the  mesenteron 
is  the  development  of  a  ventral  diverticulum, 
the  anlage  of  the  liver,  which  arises  just  caudal 
to  the  head.     This  divides  the  alimentary  canal  into  pre-  and  post- 


F I G.  208. — Reconstruc- 
tion of  alimentary  canal  of 
human  embryo,  after  His. 
al,  allantois  stalk;  cl,  cloaca; 
g,  glottis;  h,  hyoid  arch;  li, 
liver;  lu,  lung;  md,  mx,  man- 
dibular and  maxillary  arches ; 
n,  nasal  pit;  o,  omphalomesa- 
raicvein;  5,  stomach; -y,  vis- 
ceral arches;  vi,  vitelline 
stalk;  w,  Wolffian  body. 


DIGESTIVE   ORGANS.  207 

hepatic  portions  (fig.  209).  From  the  anterior  of  these  is  formed 
part  of  the  cavity  of  the  mouth  with  the  salivary  glands,  the  pharynx, 
oesophagus,  stomach,  and  duodenum;  the  post-hepatic  portion  gives 
rise  to  large  and  small  intestines,  rectum  and  cloaca,  as  well  as  to  the 
urinary  bladder.  Of  these  parts  the  pharynx  will  be  considered  in 
connection  with  the  respiratory  organs,  the  bladder  with  the  urogenital 
system.  Mouth  and  pharynx  belong  primitively  to  the  head,  but  by 
unequal  growth  the  pharynx  may  be  carried  apparently  to  some 
distance  behind  the  brain  and  other  characteristically  cephalic 
structures. 


Fig.  209. — Diagrams  of  the  alimentary  canal  in  embryos  of  6  and  8  days  of  Gymnarchus 
nitoticus,  after  Assheton.  ab,  air  bladder;  6,  early  diverticxilum  for  air  bladder;  gb,  gall 
bladder;  /,  liver;  pa,  pancreas;  pb,  posterior  part  of  air  bladder;  pc,  pyloric  caeca;  ph, 
phar}'nx;  s,  stomach. 

In  the  following  account  stress  is  laid  upon  the  epithelial  lining 
(entoderm),  the  characteristic  tissue  of  the  digestive  tract,  but  it  must 
not  be  forgotten  that  the  wall  contains  other  tissues  of  mesenchymatous 
origin.  That  part  of  the  canal  which  runs  through  the  body  cavity 
has  the  following  layers.  The  lining  epithelium  is  supported  by  a 
layer  of  connective  tissue,  containing  the  capillary  absorb tive  vessels; 
outside  of  this  are  two  layers  of  smooth  (involuntary)  muscles,  the  inner 
with  the  fibres  running  in  a  circular,  the  other  in  a  longitudinal  direc- 
tion. By  the  action  of  these  antagonistic  muscles  the  peristalsis  or 
movement  of  the  digestive  tract  is  effected,  by  which  the  food  undergoing 
digestion  is  churned  and  thoroughly  mixed  with  the  digestive  fluids, 
and  all  parts  of  it  are  brought  into  contact  with  the  absorbtive  surfaces. 
The  outer  surface  of  stomach,  intestine  and  associated  glands  is  covered 
with  the  serous  coat,  the  lining  of  the  peritoneal  cavity,  but  this  is 
lacking  from  those  parts  (pharynx,  oesophagus,  etc.)  which  are  outside 
the  region  of  the  coelom. 


208        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

THE  ORAL  CAVITY. 

The  cavity  of  the  mouth  is  limited  anteriorly  by  the  line  of  the  stom- 
odeal  involution  and  extends  back  to  the  pharynx.  It  is  lined  in  part 
by  ectoderm,  in  part  by  entoderm,  the  line  between  the  two,  as  stated 
above,  not  being  recognizable  in  the  adult.  In  the  amphibia  the  lining 
is  ciliated,  the  cilia  extending  back  to  the  stomach.  In  the  cyclostomes 
the  oral  cavity  is  funnel-shaped,  with  a  circular  or  quadrangular  open- 
ing supported  by  a  cartilaginous  ring  and  has  the  name  of  oral  hood. 
It  is  permanently  open,  there  being  no  jaws  capable  of  closure  (see 
skeleton,  p.  73)  thus  furnishing  a  marked  contrast  to  all  other  verte- 
brates in  which  there  are  jaws  and  which  are  consequently  known 
as  gnathostomes.  (Development  gives  little  support  to  the  view  that 
the  cyclostome  tongue  is  the  homologue  of  the  lower  jaw  of  the 
gnathostomes.) 

In  development  the  mouth  arises  on  the  ventral  side  of  the  head, 
some  distance  from  the  anterior  end  of  the  body.  This  position 
is  retained  throughout  life  in  most  elasmobranchs  and  in  the  sturgeons; 
but  elsewhere,  by  the  development  of  the  bony  upper  jaw  in  front  of 
the  pterygoquadrate  (p.  69)  and  the  concomitant  extension  of  Meckel's 
cartilage,  the  mouth  opening  is  gradually  transferred  to  the  anterior 
end  and  becomes  terminal. 

In  most  lower  gnathostomes  (the  holocephali  and  other  isolated 
forms  are  exceptions)  the  mouth  opening  is  bounded  by  folds  of  epithe- 
lium which  meet  when  the  mouth  is  closed.  Usually  these  folds  are 
soft  and  are  supported  below  by  connective  tissue,  but  in  birds,  turtles 
and  monotremes  they  are  comified.  It  is  only  in  the  mammals  that 
true  lips  occur.  These  are  fleshy  folds  around  the  mouth  and  their 
development  in  this  group  is  correlated  with  the  presence  of  the  dermal 
facial  muscles  (p.  134),  by  which  they  are  moved.  With  the  develop- 
ment of  lips  there  is  formed  a  space  between  lips  and  teeth,  the  vesti- 
bule of  the  mouth,  which  sometimes  (e.  g.,  some  rodents)  forms  cheek 
pouches,  lined  with  hair,  of  considerable  size. 

Teeth. 

The  primitive  function  of  the  teeth  was  apparently  to  hold  the  prey 
taken  into  the  mouth  and  this  is  their  sole  use  in  many  forms.  In 
other  species  they  have  become  efficient  organs  for  the  comminution  of 
food,  either  by  cutting  or  by  crushing  it. 


DIGESTIVE   ORGANS.  209 

There  are  two  types  of  teeth,  much  alike  in  function,  but  differing 
markedly  in  structure  and  development  and  without  genetic  relation- 
ships. The  typical  vertebrate  teeth  are  comparable  to  placoid  scales; 
they  arise  as  a  calcareous  secretion  at  the  junction  of  ectoderm  and 
mesenchyme  and  are  a  product  of  both  layers.  The  other  type  contains 
purely  cuticular  teeth,  formed  by  a  comification  of  the  epithelium  and 
have  their  analogues  in  many  invertebrates. 

True  Teeth. — The  ability  to  form  scales  is  characteristic  of  the 
skin  of  many  vertebrates.  The  primitive  type  of  these  scales  is  the 
placoid  (p.  40),  consisting  of  a  basal  portion  of  dentine  capped  with 
enamel  and  the  apex  projecting  through  the  integument  as  a  spine. 
When  invaginated  to  form  the  stomodeum  the  skin  retains  this  capacity 
of  forming  hard  structures  and  hence  any  portion  of  the  stomodeal 
walls  may  secrete  scale-like  plates.  In  fact,  in  the  teeth  of  some 
elasmobranchs  (Rata,  Mustelus,  Trygon,  etc.)  the  placoid  scale  can  be 
recognized  with  scarcely  a  modification.  In  the  ichthyopsida  teeth 
may  form  anywhere  in  the  oral  cavity  where  there  are  skeletal  parts 
— cartilage  or  bone — to  support  them.  Thus  they  may  occur,  not  only 
on  the  margins  of  the  jaws,  but  on  vomers,  palatines  and  parasphenoid, 
and  in  some  teleosts  on  the  tongue,  where  they  are  attached  to  the 
hyoid.  In  the  amniotes  -(some  squamata  excepted)  teeth  occur  only 
on  the  margins  of  the  jaws.  Teeth  are  lacking,  here  and  there,  in 
various  families  of  vertebrates  as  well  as  from  all  turtles  and  living 
birds,  but  some  extinct  birds  had  teeth.  In  the  embryos  of  both 
chelonians  and  aves  the  dental  ridge  is  formed  (vide  infra),  but  it  soon 
completely  disappears. 

In  the  development  of  a  tooth,  as  of  a  placoid  scale,  there  is 
first  a  thickening  of  the  ectoderm,  the  basal  layer  of  which  pushes  into 
the  cutis,  and  at  the  same  time  the  mesenchyme  cells  of  the  latter  layer 
multiply  beneath  the  centre  of  the  ectodermal  ingrowth,  pushing  it 
outward,  so  that  the  basal  layer  forms  a  cup  with  the  opening  toward 
the  deeper  tissues  (fig.  210).  The  mesenchyme  within  the  cup  forms 
the  dental  papilla,  while  the  ectoderm  cells  lining  the  cup  form  the 
enamel  organ.  With  farther  development  the  outer  cells  of  the  papilla 
are  converted  into  odontoblasts,  so-called  from  their  function  of  form- 
ing a  bone-like  substance,  the  dentine  or  ivory  of  the  tooth.  This, 
in  accordance  with  the  method  of  its  formation  by  secretion  from  the  ends 
of  the  odontoblasts,  has  a  prismatic  structure.  The  basal  surface  of 
the  enamel  organ  secretes  a  denser  substance,  the  enamel,  which  lies  like 
14 


2IO 


COMPARATIVE  MORPHOLOGY   OF   VERTEBRATES. 


a  cap,  firmly  united  to  the  top  and  sides  of  the  dentine.  By  continued 
additions  to  the  deeper  portions  of  the  dentine  the  tooth  is  gradually 
forced  up  through  the  epithelium  so  that  its  tip  or  crown  comes  into 
position  for  use  (eruption  of  the  tooth). 


Fig,  2IO. — Section  of  developing  tooth  germ  of  Amblystoma.     e,  epidermis;  co,  enamel 
organ;  m,  Malpighian  layer;  wc,  mesenchyme;  />,  pulp  of  tooth. 

In  the  lower  vertebrates  there  may  be  a  separate  invagination  of  the  ectoderm 
for  each  tooth,  but  in  the  mammals  there  is  a  continuous  ingrowth,  the  dental  ridge 
(fig.  21 1)  along  the  margin  of  the  jaw.  Later  this  becomes  differentiated  into 
separate  enamel  organs  and  dental  papillae,  the  separate  teeth  developing  much  as 


Fig. 


211. — Model  of  ectodermal  parts  of  jaw  of  human  embryo  40  mm.  long,  after  Rose, 
showing  the  dental  ridge  with  the  enamel  organs  for  the  first  teeth. 


in  other  groups.  From  the  posterior  side  of  this  dental  ridge  there  arises  a  continu- 
ous projection,  the  dental  shelf  (fig.  212)  which  later  gives  rise  to  the  enamel 
organs  for  the  second  or  permanent  dentition  {infra). 

The  dental  papilla  persists  throughout  life  as  the  pulp  of  the  tooth, 
continuing  to  occupy  the  space  (pulp  cavity)  in  which  it  first  appeared. 


DIGESTIVE   ORGANS. 


211 


Nerv^es  (branches  of  the  trigeminal)  and  blood-vessels  enter  the  cavity 
through  the  base  of  the  tooth.  Usually,  when  the  tooth  is  fully  formed, 
the  odontoblasts  cease  to  act,  but  exceptionally,  even  in  mammals 
(tusks  of  elephants,  incisors  of  rodents)  they  function  through  life  and 
the  tooth  continues  to  grow.  In  the  mammals  an  additional  layer  of 
modified  bone,  the  cement,  is  formed  around  the  root  of  the  tooth 
and  may  extend  on  to  the  crown. 

Just  as  the  scales  are  arranged  in  quincunx  on  the  surface  of  the 
body,  so  are  the  teeth  in  the  mouths  of  skates  and  some  other  elasmo- 


FiG.  212.  Fig.  213. 

Fig,  2 12 . — Diagram  of  germs  of  milk  and  peimanent  dentitions  in  a  mammal,  based  on 
Rose,  b,  basal  layer  of  e,  ectoderm;  dr,  dental  ridge;  ds,  dental  shelf;  eo,  enamel  organ  of 
milk  tooth;  m,  mesenchyme;  p,  pulp  of  milk  tooth;  pg,  germ  of  permanent  tooth. 

Fig.  213. — Diagrammatic  section  of  indsor  tooth,  c,  cement;  (f,  dentine;  e,  enamel; 
p,  pulp  cavity. 

branchs,  where  they  form  a  tessellated  pavement  above  and  below,  the 
teeth  being  flattened  and  used  for  crushing  the  molluscs  on  which  these 
animals  feed.  More  commonly  the  teeth  are  flattened  in  the  antero- 
posterior direction  and  have  sharp  cutting  edges.  In  such  cases,  as 
a  rule,  only  the  anterior  row  of  teeth  is  functional,  the  others  lying  folded 
down  behind,  ready  to  come  into  use  when  one  of  the  first  row  is  lost. 
Most  vertebrates  have  a  succession  of  teeth  (polyphyodont  dentition) 
and  the  elasmobranchs  show  how  this  has  come  about.  The  second 
arises  on  the  (morphologically)  posterior  side  of  the  first  and  so  on. 
In  the  non-mammalian  classes  the  number  of  such  dentitions  is  in- 
definite (polyphyodont),  but  in  the  great  majority  of  mammals  there 
are  two,  the  first  or  milk  dentition  and  the  second  or  permanent 
dentition  (diphyodont  condition). 


212        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

In  a  few  mammals  only  one  dentition  has  been  retained  (monophyodont) ;  among 
these  may  be  mentioned  the  monotremes,  sirenians  and  cetacea.  In  the  marsupial 
Myrrecobius,  where  the  permanent  dentition  is  greatly  reduced,  and  in  some  of  the 
insectivores  and  rodents,  a  prelacteal  dentition  has  been  observed  in  the  embryo, 
while  Rose  has  described  traces  of  a  prelacteal  and  a  post-permanent  dentition  in 
man.  In  a  number  of  mammals  (guinea  pigs,  many  bats,  etc.)  the  milk  dentition  is 
lost  before  birth. 


Fig.  214. — Jaws  of  a  six  month  lion,  after  Weber.     Milk  teeth  white,  permanent  dotted. 
i,  incisors;  c,  canines;  m,  molars;  p,  premolars. 

Only  a  few  fishes  (adult  Acipenser,  Coregonus,  etc.)  lack  teeth,  while 
in  most  they  extend  to  the  lining  bones  of  the  mouth  and  in  some  to 
the  hyoid  and  branchial  arches  (pharyngeal  bones) .  Usually  they  are 
conical,  but  they  may  be  flattened  and  pavement-like  or  even  form 
large  plates,  apparently  by  the  coalescence  of  numbers  of  primitive 
teeth  (dipnoi).  In  the  amphibians  the  teeth  are  not  so  widely  distrib- 
uted in  the  mouth,  occurring  on  the  margins  of  the  jaws  and  on  the 
palatines  and  vomers,  rarely  on  the  parasphenoid,  while  they  are 
entirely  lacking  in  Bufo  and  Pipa. 

Among  the  reptiles  the  turtles  and  some  of  the  pterodactyls  are 
toothless;  most  of  the  others  have  the  teeth  confined  to  the  margin  of 
the  jaws,  though  they  occur  on  the  palatines  and  pterygoids  in  the 
snakes  and  lizards,  and  rarely  (Sphenodon)  on  the  vomer.  While  the 
conical  shape  prevails,  the  teeth  present  a  great  variety  of  forms,  some  of 
the  theriomorphs  closely  simulating  the  mammals  in  their  heterodont 
dentition.     The  teeth  may  be  anchylosed  to  the  summit  of  the  jaws 


DIGESTIVE   ORGANS.  213 

(acrodont);  applied  to  their  inner  side  (pleurodont,  fig.  97,  d);  or 
have  their  roots  implanted  in  grooves  or  sockets  or  alveoli  (thecodont). 
Mention  must  also  be  made  of  the  poison  fangs  of  certain  serpents. 
These  are  specialized  teeth  borne  on  the  maxillary  bones  and  are  either 
permanently  erect  (proteroglypha)  or  the  bone  may  turn,  as  on  a  pivot, 
so  that  when  the  mouth  is  closed  the  teeth  lie  along  the  roof  of  the 
mouth,  but  when  it  is  opened,  they  are  brought  into  position  for  striking 
the  prey  (vipers,  rattlesnakes — solenoglypha) .  Correlated  with  the 
fixed  or  movable  condition  is  a  modification  in  the  teeth  themselves.  In 
the  proteroglypha  a  groove  runs  along  the  anterior  side  of  the  fang  by 


Fig.  215. — Poison  gland  and  fang  of  rattlesnake,  Crotalus  horridus.       (Princeton    1404) 
p,  poison  gland;  /,  labial  glands. 

which  the  poison  is  conducted  from  the  poison  gland  into  the  wound. 
In  the  solenoglypha  the  groove  is  rolled  into  a  tube  with  openings  near 
the  base  and  apex  of  the  tooth  (fig.  215).  In  these  solenoglyphous 
snakes  only  a  pair  of  fangs  are  functional  at  a  time,  but  there  are 
reserve  teeth  which  can  come  into  use  on  the  loss  of  the  first. 

The  greatest  variation  is  found  in  the  teeth  of  mammals,  the  heter- 
odont  dentition  being  the  rule.  Four  kinds  of  teeth  are  recognized. 
These  are  the  incisors  in  the  premaxillary  bones,  followed  by  a  single 
canine  at  the  anterior  end  of  each  maxillary  bone.  This  resembles 
the  incisors  and  differs  from  the  other  maxillary  teeth  in  its  conical  shape 
and  single  root.  Behind  the  canines  come  the  premolars  (the  bicus- 
pids of  the  dentists)  which  have  two  roots  and  complicated  crowns  and 
appear  in  both  milk  and  permanent  dentitions.  Lastly  are  the  molars, 
like  the  premolars  in  form,  with  several  roots,  but  appearing  only  in  the 
permanent  dentition.  The  corresponding  teeth  in  the  lower  jaw  have 
the  same  names. 

In  a  few  mammals,  like  the  whales,  all  of  the  teeth  are  of  a  simple 
conical  shape,  but  in  the  majority  the  crown  of  the  molars  is  marked 
by  projections — cones,   tubercles,   crests,   etc. — which   are  variously 


214 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


arranged.  When  the  teeth  are  adapted  for  cutting  they  are  called 
secodont  (cats,  fig.  214);  for  crushing, bunodont  (man);  when  mark- 
ed by  transverse  ridges,  lophodont  (elephants) ;  when  there  are  longitu- 
dinal crests,  more  or  less  crescentic  in  outline,  they  are  selenodont 
(horse,  fig.  216). 

In  the  triconodont  tooth  there  are  three  prominences  in  the  crown 
arranged  in  a  straight  line,  parallel  to  the  axis  of  the  jaw.  The  middle 
and  more  prominent  of  these  in  the  upper  jaw  is  the  protocone,  with 
a  smaller  paracone  in  front  and  a  metacone  behind.  In  the  lower 
jaw  the  corresponding  terms  are  proto-,  para-,  and  metaconid.     In 


Fig.  216. — A,  triconodont  tooth  of  Dromatherium;  B,  tri tubercular  tooth  oiSpalaco- 
therium;  C,  interlocking  of  upper  (dark)  and  lower  (light)  tritubercular  molar  teeth  (after 
Osborn);  t>,  molar  of  Erinaceus;  E,  of  horse  (selenodont  type);  c,  cingulum;  m,  metacone 
(metaconid) ;  p,  paracone  (paraconid) ;  pr,  protocone  (protoconid) ;  t,  talon. 

a  tritubercular  tooth  the  three  cones  are  arranged  in  a  triangle,  in 
such  a  way  that  they  alternate  in  the  two  jaws,  the  protocone  being  on 
the  inner  side,  the  protoconid  on  the  outer.  Tritubercular  teeth  may 
have  a  lower  projection  (talon)  on  the  hinder  side.  When  this  devel- 
ops into  a  prominent  tubercle  (hypocone,  hypoconid)  the  tooth 
becomes  quadritubercular.  Then  crests  or  lophs  may  develop, 
connecting  the  cones,  so  that  the  crown  becomes  ridged  rather  than 
tubercular. 

In  the  homodont  dentition  the  number  of  teeth  may  be  very  large,  varying 
from  100  to  200.  With  the  heterodont  dentition  the  number  is  smaller,  the  full 
dentition  in  the  placental  mammals  including  44  teeth.  From  this  number  reduc- 
tions may  occur  by  the  loss  of  teeth  of  any  kind.  The  number  of  teeth  and  of 
those  of  each  kind  is  important  in  systematic  work,  and  a  dental  fonnula  has  been 
devised  to  express  this.  As  the  number  of  teeth  in  the  two  sides  of  each  jaw  is  the 
same,  only  one  side  is  represented  in  the  formula,  while  the  teeth  of  the  upper  and 
lower  jaws  are  represented  as  fractions.  The  number  of  incisors,  canines,  pre- 
molars and  molars  of  man  are  represented  by 

•21  2  -l       .  ^,  ,.■?!  ^4 

1  ~  >  c  - ,  pm - ,  m  -;  that  of  the  opossum  by  1  - ,  c  - ,  pm  - ,  m  -. 
2123  4134 


DIGESTIVE   ORGANS. 


215 


Not  infrequently  the  enamel  is  lacking  from  the  teeth  of  mammals,  as  in  whales, 
dugongs  and  edentates,  or  it  may  be  restricted  to  one  side  of  a  tooth,  as  in  the 
incisors  of  rodents.  Sexual  differentiations  occasionally  occur  in  mammals,  certain 
teeth  (usually  canines  or  incisors,  more  rarely  premolars)  being  better  developed 
in  the  males  than  in  the  females  of  the  same  species. 

There  are  two  views  as  to  the  way  in  which  the  complicated  molars  of  the  mam- 
mals have  arisen.  Both  start  with  the  conical  tooth  as  the  primitive  condition. 
One  theory  is  that  the  fusion  of  such  simple  teeth  is  sufficient  to  account  for  the 
multiplication  of  roots  and  tubercles  in  all  of  their  varying  forms  (figs.  217,  218). 


A^^ 

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'A  AAA 

A/\AA 
AAAA 
AAA  A 
AAAA 

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^XA 


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AA 


f? 


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Fig.  217. — Diagram  of  Ae  relation  of  the  human 

Rose. 


teeth  to  the  primitive  dentition,  after 


The  other  hypothesis  is  that  parts  have  been  developed  on  the  primitive  cone, 
giving,  first,  the  triconodont  shape.  Next  these  three  cones  have  been  shifted  to 
the  tritubercular  position;  and  later  other  parts — h)rpocone,  lophs,  etc. — have 
been  added  and  these  have  been  modified  in  different  directions.  Each  view  has 
much  in  its  favor.  Embryology  is  not  at  all  decisive,  while  paleontology  favors  the 
latter  view. 


Epidermal  Teeth  occur  in  cyclostomes  and  in  larval  amphibia  and 
in  embryonic  monotremes.  In  the  cyclostomes  they  are  cones  of 
cornified  epithelium  covering  an  underlying  core  of  the  integument;  they 
are  differently  arranged  in  the  lampreys  and  myxinoids.  In  the  latter 
they  are  few,  there  being  a  single  tooth  on  the  'palate'  and  two  chev- 
ron-shaped rows  on  the  tongue.  In  the  lampreys  nearly  the  whole 
inner  surface  of  the  oral  hood  is  lined  with  these  teeth  of  varying  shape, 


2l6  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

and  there  are  a  varying  number  upon  the  tongue.  These  teeth  are 
used  as  a  means  of  fastening  the  animals  to  their  prey,  and  those  of  the 
myxinoid  tongue  are  used  for  boring  into  the  fishes  on  which  these 
animals  feed. 

In  the  larval  anura  (the  larval  Siren  is  said  to  resemble  them)  the 
edges  of  the  jaws  are  armed  with  cornified  papillae,  serving  as  teeth,  the 
arrangement  of  which  varies  in  different  genera.  They  are  frequently 
aggregated  in  dental  plates,  used  in  scraping  the  algae  from  submerged 
objects.     They  are  not  related  to  the  teeth  of  cyclostomes. 

In  the  embryo  monotreme  teeth  are  formed  as  in 
other  mammals,  of  a  multituberculate  type,  with  a 
normal  enamel  organ  (fig.  219),  but  these  are  lost 
before  birth.  During  their  eruption  the  adjacent  epi- 
dermis becomes  cornified,  gradually  extends  beneath 


v,w^;;3v™w;mw     ,,^^^1^^.^    ^^^©2x 
<^^^.    |(^^/>  ^£v__^,^ 

Fig.  218.  Fig.  219. 

Fig.  218. — Teeth  of  Chlamydoselache  (after  Rose)^  showing  a  triconodont  tooth  arising 
from  the  fusion  of  three  simple  teeth. 

Fig.  219. — Diagram  of  development  of  teeth  in  Ornithorhynchus,  after  Thomas  and 
Poulton.  a,  tooth  covered  with  enamel  organ,  beneath  oral  epitheUum;  h,  just  before 
eruption;  c,  tooth  erupted;  d,  edges  of  epithelium  cornified;  e,  horny  plate  formed,  contains 
the  tooth;/,  tooth  lost,  plate  separated  from  its  surroundings. 

each  tooth  and  after  the  loss  of  the  true  tooth  this  forms  a  horny 
plate,  used,  like  those  of  many  birds,  in  holding  and  crushing  the 
food. 

In  this  connection  mention  may  be  made  of  the  baleen  or  'whale- 
bone' of  the  balenid  whales.  This  takes  the  form  of  large  plates  of 
horny  material,  attached  in  series  to  the  margins  of  the  upper  jaw,  so 
that  with  their  fringed  ends  and  edges  they  serve  as  strainers  to  extract 
the  plankton  (minute  floating  life)  from  the  sea.  This  baleen  is  formed 
by    the    agglutination    of    enormously    developed    cornified    papillae. 

Egg  Teeth. — In  the  embryos  of  certain  lizards  and  snakes  one  of  the  median 
teeth  of  the  first  dentition  of  the  premaxillary  region  projects  from  the  mouth  and 
is  used  for  the  rupture  of  the  egg  shell,  thus  allowing  the  escape  of  the  young. 
In  the  turtles,  Sphenodon,  crocodiles,  birds,  and  monotremes  an  egg  tooth  is  formed 
on  the  upper  surface  of  the  beak  which  is  used  for  the  same  purpose.  However,  it 
differs  greatly  as  it  is  but  a  thickening,  often  calcified,  of  the  epidermis  (Fig.  195).- 


DIGESTIVE   ORGANS.  217 

The  Tongue. 

The  tongue  as  it  occurs  in  its  more  primitive  condition  in  the  fishes 
is  merely  a  fleshy  fold  developed  from  the  floor  of  the  mouth  between  the 
hyoid  and  mandibular  arches,  the  hyoid  frequently  extending  into  and 
supporting  it.  It  is  incapable  of  motion,  except  as  moved  by  the  sup- 
porting skeleton,  for  it  lacks  intrinsic  muscles.  It  is  sensory,  having 
both  tactile  and  gustatory  functions.  It  is  often  papillose,  and  in  a 
few  teleosts  it  bears  teeth  (p.  209). 

The  tongue  in  the  cyclostomes  is  considerably  different.  Here  it 
is  thick  and  fleshy  and  is  supported  by  a  cartilaginous  skeleton  (p.  75) 
and  is  moved  by  appropriate  protractor  and  retractor  muscles  at  the 
base,  developed  from  the  postotic  myotomes  and  innervated  by  the 
hypoglossal  nerve.  With  its  terminal  armament  of  epidermal  teeth 
it  ser\' es  as  the  boring  organ  with  which  the  myxinoids  obtain  entrance 
into  their  prey,  while  in  the  lampreys  it  serves  as  a  rasping  organ  and 
also  as  part  of  the  sucking  apparatus. 

In  the  amphibia  there  is  a  greater  range  of  structure.  In  a  few 
anura  (aglossa)  the  tongue  is  practically  absent;  in  the  perennibranchs 
it  is  scarcely  more  advanced  than  in  the  fishes,  but  elsewhere  it  contains 
intrinsic  muscles  and  is  extremely  mobile.  It  consists  of  a  small  basal 
portion  corresponding  to  the  tongue  of  the  fish,  to  which  is  added  a 
large  glandular  part  arising  between  the  copula  and  the  lower  jaw. 
This  secretes  the  slime,  so  useful  in  capturing  the  prey.  In  the  anura 
the  tongue  is  attached  at  the  margin  of  the  jaw,  its  free  end,  when  at 
rest,  being  folded  back  on  the  floor  of  the  mouth.  In  urodeles  the  base 
of  attachment  is  more  extensive  and  embraces  the  anterior  margin  of 
the  tongue  and  part  of  the  ventral  surface  as  well.  The  supporting 
skeleton  (fig.  85)  consists  of  the  median  portion  (copula)  with  usually 
two  pairs  of  cornua,  largely  formed  from  the  ventral  ends  of  the  hyoid 
and  first  branchial  arches  (see  p.  64). 

The  reptilian  tongue  includes  not  only  the  parts  found  in  the  am- 
phibia (the  fold  above  the  basihyal),  but  also  a  median  growth,  the 
tuberculum  impar,  arising  between  the  basihyal  and  the  lower  jaw, 
and  also  a  pair  of  lateral  folds  lying  above  the  first  visceral  arch 
(Lacerta).  In  the  turtles  and  crocodiles  the  tongue  lies  on  the  floor 
of  the  mouth  and  is  not  protrusible.  In  the  squamata  it  can  be  extended 
from  the  mouth,  and  in  snakes  and  many  lizards  there  is  a  sheath  into 
which  it  is  withdrawn.     In  many  snakes  the  tongue  is  two-pointed  at  the 


2l8 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


tip;  in  the  lizards  its  shape  varies  greatly,  the  differences  being  used  in 
classifying  these  animals.     In   the   reptiles  (fig.  220)  with  retractile 

tongue  the  hyoid  apparatus  extends  into 
the  tongue,  its  unpaired  anterior  portion 
being  called  the  os  entoglossum  (copula 
or  basihyal),  while  the  two  cornua  (usually, 
hyoid  and  first  branchial)  afford  attachment 
for  the  retractor  muscles.  In  addition  to 
the  usual  lingual  nerve  (glossopharyngeal) 
the  tongue  also  receives  a  lingual  twig  from 
the  mandibular  branch  of  the  fifth  nerve. 

In  birds  the  tongue  has  lost  the  lateral 
parts  of  the  reptilian  tongue  and  with  this 
the  trigeminal  branch.  It  contains  no  in- 
trinsic muscles.  In  its  form  it  varies  greatly, 
but  usually  it  is  slender  and  is  covered  with 
retrorse   papillae.     Its   skeleton  is  also  re- 

FiG.  220.-Hyoid  apparatus  of  ^^^^^    (^g-   ^^l)   ^^^    COnsistS  of  an  OS  en- 

Heloderma,  after  Cope,     b,  first  toglossum,  bearing  in  front  a  pair  of  ele- 

branchial;  c,  copula:  h.  hyoid.  ,        ,  .  .  ,  i  .  i 

ments  (paraglossae)  and  on  the  sides  a 
pair  of  cornua  (first  branchials)  and  in  the  median  line  behind,  a 
urohyal  portion.  This  skeleton  has  a  marked  development  in  the 
woodpeckers,  where  the  cornua  curve  around  the  base  of  the  skull 


Fig.  221. — ^Two  stages  in  developing  tongue  and  pharyngeal  floor  of  man,  after  His. 
c,  copula  (basihyal  element);  cs,  cervical  sinus;  ep,  epiglottis;  g,  glottis;  h,  hyoid  arch;  m, 
mandibular  arch;  mth,  median  anlage  of  thyreoid;  t,  tuberculum  impar;  tg,  tongue. 

and  over  its  dorsal  side  to  the  neighborhood  of  the  nostril,  a  condi- 
tion correlated  with  the  use  of  the  tongue  in  these  animals. 


DIGESTIVE  ORGANS. 


219 


In  the  whales  the  tongue  has  little  power  of  motion,  but  elsewhere 
in  the  mammals  it  is  very  mobile,  reaching  the  extreme  in  the  ant-eaters. 
This  mobility  is  largely  due  to  the  extensive  intrinsic  musculature. 
The  tongue  is  developed  from  the  tuberculum  impar,  which  furnishes 
the  larger  anterior  part  (fig.  221),  the  rest  arising  from  the  fleshy  ridges 
above  the  hyoid  arch.  In  the  adult  the  line  between  these  parts  is  largely 
obliterated,  but  it  lies  near  the  line  of  circumvallate  papillae  (p.  189) 
and  the  foramen  caecum,  a  blind  tube  connected  with  the  development 
of  the  thyreoid  gland.  Arising  in  this  .,__  S 
way  from  the  tubercle  and  the  lateral  supra- 
hyoid parts,  the  tongue  of  the  amphibia  is 


Fig.  222.  Fig.  223. 

Fig.  222. — \^entral  and  side  views  of  tongue  of  Stenops  gracilis,  after  Weber.  /, 
lateral  margin  of  sublingua;  m,  plica  mediana. 

Fig.  223. — Section  through  lyssa  of  late  dog  embryo,  after  Nussbaum.  c,  cartilage  of 
lyssa,  cl,  capsule  of  lyssa;  m,  muscles  of  tongue;  ml,  longitudinal  and  transverse  muscles 
of  lyssa;  s,  septum  of  tongue. 


unrepresented  in  that  of  most  mammals,  unless  it  be  in  the  sublingua, 
a  fleshy  fold  developed  beneath  the  functional  tongue  in  the  marsupials 
and  lemurs  (fig.  222).  Traces  of  this  are  to  be  found  in  other  mam- 
mals, even  in  man,  as  folds  (plicse  fimbriatae)  beneath  the  tongue. 
In  some  cases  {Stetwps)  this  sublingua  is  supported  by  a  cartilage 
which  is  regarded  as  an  entoglossal  part.  Others  think  that  the 
tongue  of  the  lower  vertebrates  is  represented  in  the  mammalian 
tongue  and  regard  the  lyssa  as  the  os  entoglossum.  The  lyssa  is 
a  vermiform  structure  of  cartilage,  muscle  and  connective  tissue  (fig. 
223)  lying  ventral  to  the  septum  of  the  tongue. 

The  tongue  varies  considerably  in  shape  in  the  different  mammalian 
orders,  but  the  differences  are  of  little  morphological  importance. 


220 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  dorsal  surface  is  usually  covered  with  a  soft  epithelium,  developed 
into  papillae  of  varying  shapes,  some  being  sensory  in  character,  and 
some  are  occasionally  (monotremes,  felidae)  cornified. 

The  skeleton  of  the  mammalian  tongue  (hyoid  apparatus)  varies 
considerably.  In  its  most  complete  development  it  consists  of  a  body 
(copula  of  the  hyoid  and  first  branchial)  in  the  median  line,  which  bears 
two  pairs  of  cornua.  The  anterior  pair  (lesser  horns  of  human 
anatomy)  are  usually  elongate,  and  consist  of  a  series  of  ossicles  (p.  loi) 
connecting  the  body  with  the  otic  region  of  the  skull.  The  second 
pair  (greater  cornua  of  man)  are  occasionally  absent.  In  man  the 
greater  part  of  the  anterior  cornua  is  represented  by  the  stylohyoid 
ligament,  the  proximal  portion  being  fused  to  the  skull  as  the  styloid 
process. 


Oral  Glands. 

In  the  cyclostomes  there  is  a  large,  so-called  'salivar}'  gland'  of 
unknown  function,  opening  into  the  mouth  on  either  side  below  the 
tongue.  With  this  exception,  glands  are  lacking  from  the  mouths  of 
aquatic  ichthyopsida.  With  the  assumption  of  pulmonate  respiration 
and  more  terrestrial  habits,  the  mouth  is  no  longer  constantly  bathed 

with  water  and  so  glands  appear, 
increasing  in  number  and  com- 
plexity in  the  higher  forms.  The 
secretion  of  these  glands  aids  in 
moistening  the  food,  and  not  in- 
frequently it  is  adhesive  and  is 
used  in  capturing  the  prey.  In  the 
mammals  true  salivary  glands  ap- 
pear. The  saliva  secreted  by  them 
contains  not  only  mucus,  but  also  a 
digestive  ferment  (ptyalin)  which 
changes  starch  into  sugar.  The 
names  of  the  various  oral  glands 
(labial,  buccal,  lingual,  retrolingual,  etc.)  are  roughly  indicative  of 
their  position. 

In  the  terrestrial  amphibia,  snakes  (fig.  215)  and  lizards  there  are 
labial  glands,  opening  at  the  bases  of  the  teeth,  and  an  intermaxil- 
lary or  internasal  gland  in  the  septum  between  the  nasal  cavities,  as 
well  as  palatal  glands  near  the  choanae  (the  internasal  gland  is  lacking 


Fig.  224. — Transverse  section  of  tongue 
and  lower  jaw  of  Lacerta,  after  Gegenbaur. 
d,  tooth ;/j,  hyoid  cartilage;  /,  labial  glands; 
w,  muscles;  si,  sublingual  gland;  t,  tongue. 


DIGESTIVE   ORGANS. 


221 


in  the  caecilians) .    Many  reptiles  also  have  a  sublingual  gland  on  either 

side  (fig.  224).     In  many  snakes  a  pair  of  the  labial  glands  are  greatly 

developed  and  have  migrated  into  the  zygomatic  ligament,  where  they 

have  become  modified  into  the  well-known  poison  glands  (fig.  215), 

the  ducts  of  which  connect  with  the  poison  fangs 

(p.  213).     In  the  only  known  poisonous  lizards 

{Heloderma)  the  sublingual  glands  furnish  the 

poison.     Oral    glands  are  poorly  developed  in 

the  sea  turtles  and  the  crocodilians. 

►'•    Birds  lack  the  labial  and  internasal  glands, 

but  they  have  numerous  other  glands  opening 

separately  into  the  roof  of  the  mouth  (fig.  225) 

as  well  as  anterior  and  posterior  sublinguals  and 

frequently  an  'angle  gland'  at  the  angle  of  the 

mouth,  which  may  be  the  last  remnant  of  the 

labial  glands  of  the  other  Sauropsida. 

Besides  numerous  smaller  glands  (labials, 
buccals,  Unguals,  palatines)  imbedded  in  the 
mucous  membrane  and  opening  separately  into 
the  mammalian  mouth,  the  salivary  glands, 
though  absent  from  the  cetacea,  form  a  distin- 
guishing feature  of  the  group.  These  salivary 
glands  are  usually  in  the  neighborhood  of  the 
mouth,  but  one  or  more  of  them  may  be  carried 
back  into  the  neck  (fig.  226),  but  in  all  cases  the 
homologies  are  decided  by  the  openings  of  the 
ducts.  The  salivary  glands  include  the  sub- 
maxillary and  sublingual  of  the  lower  groups,  and  in  addition  the 
parotid  gland,  apparently  a  development  within  the  class.  The  sub- 
maxillary normally  lies  in  the  lower  jaw  beneath  the  mylohyoid 
muscle,  and  its  duct  (Wharton's  duct)  opens  near  the  lower  incisor 
teeth.  Near  this  is  frequently  a  retrolingual  gland,  its  duct  open- 
ing near  the  former.  The  sublingual  gland  occurs  between  the  tongue 
and  the  alveolar  margin  of  the  lower  jaw  and  usually  empties  by 
numerous  duct.  The  parotid  gland  has  its  normal  position  near  the 
ear  and  its  ducts  (Stenon's  duct)  pours  the  secretion  out  near  the 
molars  of  the  upper  jaw.  Other  oral  glands  are  occasionally  present, 
like  the  molar  glands  of  ungulates  and  the  orbital  glands  of  dogs, 
both  of  which  have  ducts  leading  into  the  mouth. 


Fig.  225. — ^Palatal  sur- 
face of  hen,  after  Heid- 
rich.  ch,  anterior  end  of 
choana;  gs,  openings  of 
sphenopterygoid  glands; 
in,  infundibular  opening; 
Ip,  mp,  openings  of  lat- 
eral and  medial  palatine 
glands;  m,  opening  of 
gl.  maxillaris  monosto- 
matica. 


222 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


PHARYNX. 

The  pharynx  is  the  division  of  the  alimentary  canal  intervening 
between  the  cavity  of  the  mouth  and  the  oesophagus  and  is  characterized 
by  being  at  once  alimentary  and  respiratory.  From  its  walls  are  devel- 
oped the  gill  clefts  and  lungs  as  well  as  a  number  of  derivatives  of 
these,  and  it  also  receives  the  internal  openings  of  the  nasal  passages. 
Hence  it  is  best  described  in  connection  with  the  respiratory  system. 


Fig.  226. — Salivary  glands  of  fruit  bat,  Pteropus  cotnpicillatus  (Princeton,  2065). 
P,  pd,  parotid  gland  and  duct;  rl,  rid,  retrolingual  gland  and  duct;  sm,  smd,  submaxillary 
gland  and  duct. 


THE  (ESOPHAGUS. 

That  part  of  the  digestive  tract  between  the  pharynx  and  the 
entrance  of  the  bile  duct  (fig.  209)  develops  into  oesophagus,  stomach 
and  that  part  of  the  intestine  known  as  the  duodenum.  Stomach  and 
duodenum  are  separated  by  the  pyloric  valve  described  below,  but  it 
is  difficult  to  draw  a  clear  line  between  oesophagus  and  stomach.  In 
general  it  may  be  said  that  the  oesophagus  is  the  tract  immediately 
succeeding  the  pharynx,  lying  in  front  of  the  body  cavity  and  thus 
lacking  a  serous  coat;  that  it  is  smaller  than  the  stomach,  and  that 
there  are  no  digestive  glands  in  its  walls;  but  all  of  these  statements 
have  exceptions. 


DIGESTIVE  ORGANS.  223 

The  oesophagus  varies  in  length  with  the  length  of  the  neck  of  the 
animal,  being  short  in  the  ichthyopsida,  longer  in  the  reptiles,  and 
reaching  its  extreme  in  the  birds.  In  some  its  internal  lining  epithelium 
is  smooth,  but  more  commonly  it  bears  longitudinal  folds,  while  in  the 
chelonians  it  is  provided  with  comified  papillae,  pointing  backward. 
Outside  of  the  epithelium  its  walls  contain  muscles,  those  at  the 
cephalic  end  being  striped  and  these  may  extend  back,  in  some  in- 
stances, even  on  to  the  stomach.  They  are  apparently  derivatives 
of  the  pharyngeal  region.  Usually  the  oesophagus  is  of  the  same  di- 
ameter throughout,  but  frequently  in  birds  it  has  a  marked  dilatation, 
the  ingluvies  or  crop.  This  may  be  an  expansion  of  one  side  of  the 
tube,  or,  as  in  pigeons,  it  may  consist  of  a  median  and  a  pair  of  lateral 
chambers.  The  extreme  of  development  of  the  crop  occurs  in  Opis- 
thocomusj  where  the  organ  is  extremely  muscular  and  has  numerous 
longitudinal  folds. 

The  crop,  which  is  usually  supported  by  the  furcula,  may  be  either 
a  resen^oir  for  food,  or  it  may  be  a  glandular  organ,  its  secretions 
sending  to  moisten  the  food  or  even  to  initiate  its  digestion.  In  the 
pigeons  at  the  breeding  season  the  secretion  is  a  milky  fluid  and  is 
used  in  feeding  the  young. 

THE  STOMACH. 

The  stomach  is  apparently  a  new  acquisition  in  the  vertebrates, 
possibly  arising  as  a  place  for  the  storage  of  food.  This  view  is  sup- 
ported by  several  facts.  In  the  embryo  vertebrate  and  in  the  adult  of 
Amphioxus  the  duct  from  the  liver  immediately  follows  the  pharynx, 
opening  just  behind  the  last  gill  cleft;  while  the  innervation  from  the 
tenth  nerve  shows  that  both  stomach  and  oesophagus  are  parts  of  the 
pharynx  greatly  drawn  out  (fig.  209). 

The  pylorus,  which  limits  the  stomach  behind,  is  a  fold  of  the 
lining  mucous  membrane  projecting  into  the  interior  and  reinforced 
by  a  circular  (sphincter)  muscle,  which  by  its  contraction,  closes  the 
tube  so  that  no  food  can  pass  from  the  stomach  until  it  is  properly 
acted  upon  by  the  gastric  fluids.  The  anterior  end  of  the  stomach  is 
not  so  well  marked.  Usually  it  is  differentiated  from  the  oesophagus 
by  its  greater  diameter,  but  in  some  of  the  fishes  (fig.  227,  a)  there 
is  no  distinction  in  size.  The  stomach  lies  in  the  coelom  and  hence  is 
covered  externally  by  the  serous  membrane   (peritoneum),  but  the 


224 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


oesophagus  usually  extends  a  short  distance  into  the  body  cavity  and 
then  its  lower  end  has  the  same  coat. 

The  true  stomach  is  characterized  by  the  presence  of  glands,  de- 
veloped from  the  mucous  layer  and  emptying  into  the  lumen.  Of 
these  glands  there  are  at  most  (mammals)  three  kinds:  cardiac,  near 
the*  entrance  of  the  oesophagus,  which  secrete  an  albuminoid  fluid; 


Fig.  227. — Dififerent  shapes  of  stomachs,  mostly  after  Nuhn  (Keibel).  a,  Belone;  h, 
Proteus;  c,  Tropidonotus  natrix;  d,  Gobius;  e,  shark;  f,  Phoca  vitulina;  g,  Polypterus;  h, 
Fulica  atra;  i,  Testudo  grosca;  k,  land  tortoise;  /,  rabbit;  w,  pig;  n,  owl;  0,  crocodile;  p, 
Delphinus;  q,  Halmaturus. 

pyloric,  near  the  pylorus,  which  form  mucus;  and  the  most  character- 
istic, the  fundus  glands,  which  secrete  a  digestive  ferment,  pepsin. 
(For  the  structure  of  these  glands  reference  should  be  made  to  histological 
text-books.)  Tested  by  glands,  many  vertebrates  (dipnoi,  cyprinoids) 
lack  a  true  stomach,  while  the  sturgeons  have  the  gastric  glands  extend- 
ing into  the  oesophagus.  On  the  other  hand,  a  part  of  the  enlargement 
called  the  stomach  in  mammals  often  includes  a  part  of  the  oesophagus 
(fig.  228,  A,  E), 


DIGESTIVE   ORGANS.  225 

The  shape  of  the  stomach  is  to  some  extent  dependent  upon  the 
shape  of  the  body.  In  the  elongate  species  it  lies  in  the  axis  of  the 
trunk,  especially  in  the  lower  vertebrates  (fig.  227,  a),  but  with  increase 
in  the  body  width  it  becomes  more  transverse.  This  involves  a  bending 
and  a  torsion  of  the  tube,  always  to  the  right,  and  results  in  two  faces 
or  *  curvatures,'  a  lesser  or  anterior,  and  a  greater  or  posterior,  the 
greater  curvature  often  expanding  into  a  so-called  fundus  region. 
The  end  of  the  stomach  which  connects  with  the  oesophagus  is  nearest 
the  heart  and  hence  is  called  the  cardiac  end. 

In  the  fishes  the  stomach  may  be  either  straight  or  saccular,  often  assuming  the 
form  of  a  blind  sac  (fig.  22 y,  g).  The  line  between  oesophagus  and  stomach  is  not 
well  marked,  as  the  oesophageal  folds  may  continue  into  the  stomach.  The  teleosts 
exhibit  the  greatest  variety  in  shape,  in  correlation  to  the  differences  in  food.  All 
gastric  glands  are  lacking  in  the  cyprinoids,  while  Amia  has  both  cardiac  and  pyloric 
glands,  and,  like  many  teleosts,  the  stomach  is  ciliated.  In  the  amphibians  and 
reptiles  the  distinctions  between  oesophagus  and  stomach  are  more  marked,  most 
in  the  crocodiles.  In  the  amphibians  the  ciliation  of  the  mouth  is  continued  into 
the  stomach. 

In  the  birds  there  is  a  differentiation  of  the  gastric  region  into  two 
regions,  an  anterior  glandular  stomach  or  proventriculus,  and  a  pos- 
terior muscular  gizzard.  The  proven tricular  glands  secrete  a  diges- 
tive fluid,  and  the  food,  mixed  with  this,  is  passed  on  to  the  gizzard. 
The  walls  of  the  latter  have  their  muscles  developed  into  a  pair  of  discs 
with  tendinous  centres,  while  the  glands  of  the  gizzard  form  a  secretion 
which  hardens  into  a  horny  (keratoid)  lining,  sometimes  developing 
into  tubercular  structures,  of  great  use  in  grinding  the  food,  thus  in  part 
making  good  the  absence  of  teeth.  In  the  grain-eating  birds  small 
pebbles  are  taken  into  the  gizzard  and  are  used  in  triturating  the  food. 
(In  the  fossil  pterodactyls  small  clusters  of  stones  are  sometimes  found 
in  such  a  position  as  to  lead  to' the  supposition  that  these  reptiles  also 
had  a  gizzard.)  The  gizzard  is  best  developed  in  the  grain-eating 
birds  and  is  weakest  in  the  birds  of  prey.  In  one  species  of  pigeon 
part  of  the  wall  of  the  gizzard  is  ossified. 

The  mammalian  stomach  shows  the  greatest  range  of  form  (figs. 
227,  228)  and  the  greatest  development  of  different  kind  of  glands. 
It  may  be  a  simple  sac  or  it  may  be  subdivided  into  a  series  of  chambers. 
It  may  be  almost  wholly  oesophageal  in  character  {Ornithorhynchus, 
fig.  228,  A).  Occasionally  the  cardiac  glands  may  be  absent.  It 
may  be  a  simple  sac,  longitudinal  or  transverse  in  position,  or  it  may  be 
IS 


226 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


Fig.  228. — Outlines  of  the  stomachs  of  various  mammals  (various  authors),  after 
Oppel,  to  show  the  distribution  of  the  different  glandular  regions.  Horizontal  lines, 
oesophageal;  oblique,  cardiac;  dots,  fundus;  crosses,  pyloric;  A,  Ornithorhynchus;  B, 
gray  rat;  C,  tapir;  Z),  seal;  E,  whale  (Lagenorhynchus) ;  F,  mouse;  G,  dog;  H,  kangaroo 
(Macropus). 


Fig.  229. — Diagram  of  ruminant  stomach,  the  dotted  line  showing  the  course  of  the 
food,  a,  abomasum;  oe,  oesophagus;  p,  pylorus;  fs,  psalterium  (omasus,  manyplies);  tr^ 
reticulum  (honeycomb);  ru,  rumen  (paunch). 


DIGESTIVE   ORGANS.  227 

divided  into  chambers,  the  division  reaching  its  extreme  in  the  rumi- 
nants (fig.  229)  and  the  cetacea  (fig.  228,  E)  where  four  compart- 
ments can  be  recognized.  In  the  ruminants  two  of  these,  the  mmen 
or  paunch  and  the  reticulum  or  honey-comb  are  expansions  of  the 
oesophagus  and  serve  as  reservoirs  for  food  lDef ore  its  complete  mastica- 
tion, after  which  it  follows  the  course  of  the  dotted  lines  to  the 
psalteriimi,  omasus  or  manyplies  and  the  abomasus  or  rennet 
stomach  for  gastric  digestion. 

INTESTINE. 

The  remainder  of  the  pre-hepatic  portion  of  the  alimentary  canal, 
the  duodenum,  extending  from  the  pylorus  to  the  entrance  of  the  bile 
duct,  is  considered  as  part  of  the  intestine.     It  is  especially  noticeable 


Fig.  230. — Digestive  tube  of  garpike,  Lepidosteus  (after  Gegenbaur).     i,  small  intestine; 
oe,  cesophagus;  pc,  pyloric  caeca;  pg,  pylorus;  r,  rectum;  s,  stomach;  sv,  spiral  valve. 

in  many  ganoids  and  teleosts  (figs.  230,  233)  where  it  may  bear  from 
one  to  two  hundred  blind  digestive  tubes,  the  pyloric  caeca.  The 
same  region  in  a  few  elasmobranchs  may  have  a  pair  of  these  caeca  or 
(Galeus)  it  may  be  expanded  into  a  pouch  ('bursa  Entiana'). 

The  post-hepatic  intestine  is  the  seat  of  most  of  the  digestive  pro- 
cesses and  of  absorption  of  the  products  of  digestion.  Here  the  food, 
coming  from  the  stomach,  is  mixed  with  the  bile  from  the  liver  and 
with  the  pancreatic  juice  and  with  the  secretions  of  numerous  small 
glands  in  the  intestinal  wall.  The  increase  of  surface  needed  for  ade- 
quate digestion  and  absorption  is  provided  in  several  ways.  There 
may  be  an  elongation  of  the  tube  which  results  in  its  becoming  coiled  in 
the  body  cavity;  the  mucous  lining  may  develop  folds,  both  longitudinal 
and  circular;  or  the  folds  may  break  up  into  numerous  minute,  finger- 
like processes  (villi)  which  give  the  surface  a  velvety  appearance.  The 
food  undergoing  digestion  is  moved  back  and  forth  (peristaltic  mo- 
tion) by  the  antagonistic  action  of  the  muscles  of  the  intestinal  wall 
(p.  207),  bringing  all  of  it  in  contact  with  the  absorb tive  surface. 


228 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  length  of  the  intestine  is  roughly  related  to  the  food,  being 
longer  in  the  plant-eating  than  in  the  carnivorous  species.  This  is 
strikingly  shown  in  the  frogs,  where  the  tadpole  (larva)  has  a  very 
long  intestine,  correlated  with  the  vegetable  food,  while  the  adult 
flesh-eating  frog  has  a  canal  hardly  longer  than  that  of  the  tadpole  of 
half  the  size. 

In  the  intestine  there  are  two  divisions,  an  anterior  small  intestine 
and  a  posterior  large  intestine,  terms  adapted  from  the  digestive  tract 
of  man,  though  not  always  appropriate  in  the  lower  groups.  The 
line  between  the  two  may  be  marked  externally  by  the  development  of 


Fig.  231.- — Spiral  valve  of  Raia,  after  Mayer. 

one  or  two  blind  pouches  or  caeca  at  their  junction  or  by  a  circular 
fold  or  a  pair  of  internal  folds  of  the  lining,  constituting  an  ileo-colic 
(ileo-caecal)  valve,  both  valve  and  caeca  coexisting  in  many  cases. 
Both  large  and  small  intestines  may  be  subdivided,  chiefly  by  differ- 
ences in  their  walls.  Thus  in  the  small  intestine 
there  may  be  recognized  in  different  groups  a 
jejunum,  a  spiral  valve  region  and  an  ileum, 
while  the  large  intestine  may  furnish  a  colon,  a 
rectmn  and  a  cloaca. 

In  the  cyclostomes  but  two  regions  occur, 
the  intestine  and  the  rectum,  differentiated  ex- 
ternally by  the  larger  size  of  the  latter.  In  the 
petromyzonts  there  is  an  internal  fold  of  the  in- 
testine which  pursues  a  slightly  spiral  course, 
constituting  a  spiral  valve,  a  structure  which 
reaches  its  highest  development  in  the  elasmobranchs. 

In  the  elasmobranchs  the  intestine  is  nearly  straight,  but  its  dif- 
ferentiation has  proceeded  farther.  At  the  junction  of  small  and  large 
intestine  is  a  dorsal  blind  sac,  the  rectal  gland.     Its  function  is  un- 


FiG.  232. — Diagram 
of  spiral  valve  of  Carcha- 
rias. 


DIGESTIVE   ORGANS. 


229 


known,  but  it  apparently  corresponds  to  the  caeca  of  the  higher  groups. 
In  the  '  small '  intestine  is  the  spiral  valve  which  has  two  forms,  both 
leading  to  increase  of  surface.  In  most  species  a  fold,  carrying  blood- 
and  lymph-vessels,  arises  in  a  spiral  line  from  the  wall  of  the  tube,  and 
its  free  edge  projects  into  the  lumen  like  a  spiral  stairway  (fig.  231). 
In  a  few  forms  (Carchariidae,  Galeocerdo)  the 
line  of  origin  of  the  fold  is  straight  and  its  free 
margin  is  coiled  like  a  roll  of  paper  (fig.  232). 
In  the  large  intestine  rectum  and  cloaca  are 
recognized,  the  cloaca  being  that  part  which 
receives  the  ends  of  the  excretory  and  repro- 
ductive ducts  and  thus  is  both  digestive  and 
urogenital  in  character. 

Ganoids  and  dipnoi  (figs.  230,  233)  also  have  the 
intestine  nearly  straight  and  a  spiral  valve,  least 
developed  in  Lepidosteus.  In  the  teleosts  the  canal 
may  be  straight  (fig.  227)  or  may  make  more  or  fewer 
coils,  the  predaceous  species  being  simplest,  while  in 
the  mullet  (MugiT)  there  may  be  13  or  14  turns.  In 
the  teleosts  the  line  between  small  and  large  intestine 
is  often  marked  by  an  ileo-colic  valve  and  a  few  species 
have  a  caecum  or  rectal  gland.  A  spiral  valve  rarely 
occurs  in  teleosts  and  a  cloaca  is  never  found.  In  a 
few  teleosts,  in  correlation  with  the  translation  of  the 
ventral  fins,  the  anus  may  lie  in  front  of  the  pectoral 
girdle. 

The  intestine  is  straight  in  the  caecilians,  has  a 
few  coils  in  the  perennibranchs  and  more  in  the  sala- 
manders, while  the  anura  have  a  greatly  convoluted 
intestine.  (Reference  has  already  been  made  to  the 
differences  between  the  intestines  of  the  larval  and 
adult  frogs  (p.  228).  The  line  between  small  and 
large  intestine  is  frequently  marked  in  the  amphi- 
bians by  an  ileo-colic  valve  and  in  a  few  forms 
(Rana,  Salamandra)  there  is  a  rudimentary  caecum. 
The  rectum  is  larger  than  the  rest  of  the  intestine  and  a  cloaca  is  always  present 
in  the  amphibia. 

The  reptiles  have  the  intestine  coiled  (nearly  straight  in  amphisbaenans)  and 
usually  of  about  the  same  diameter  throughout.  Small  and  large  intestine  are 
separated  by  an  ileo-colic  valve,  and  except  in  crocodiles  a  caecum  is  usually  present, 
while  a  cloaca  constandy  occurs.  The  spirally  twisted  coprolites  of  the  ichthyo- 
saurs  have  been  supposed  to  indicate  the  existence  of  a  spiral  valve,  but  since  in 
other  groups  the  faeces  are  formed  in  the  rectum,  this  is  not  conclusive. 


Fig.  233. — Digestive  tract 
of  soup  {Stenostomus  chrysops 
— ^Princeton  296).  bd,  bile 
duct;  gb,  gall  bladder;  I,  liv- 
er; It, '  large  intestine;  pc, 
pyloric  caeca;  si,  small  in- 
testine. 


230 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  intestine  is  longer  in  the  birds  than  in  the  reptiles,  but  there  is  considerable 
difference  in  the  group  in  this  respect.  The  great  increase  comes  in  the  colon  which 
is  coiled  in  different  ways,  which  may  be  reduced  to  seven  plans  or  combinations 
of  loops  and  spirals  (fig.  234).     In  a  few  forms  (woodpeckers,  parrots,  etc.)  there 


Fig.  234. — Types  of  coiling  of  the  intestines  of  birds,  after  Gadow.  A,  isocoelous; 
B,  anticoelous;  C,  antipericoelous ;  D,  isopericoelous;  E,  cyclocoelous;  F,  plagiocoelous ;  G, 
telogyrous;  p,  pylorus. 

is  no  caecum,  but  usually  the  junction  of  large  and  small  intestine  is  marked  by  one 
or  two  caeca  (fig.  235).  In  some  cases  these  caeca  are  lined  with  villi,  or  portions 
may  be  ciliated,  while  the  very  large  caecum  of  the  ostrich  is  spirally  coiled.  Many 
birds  have  a  pocket,  the  bursa  Fabricii,  of  unknown  functions,  developed  from  the 


Fig.  235. — Alimentary  canal  of  Chauna,  after  Mitchell,  c,  caeca;  /,  large  intestine;  p 
proventriculus;  pv,  portal  vein;  rv,  rectal  vein;  s,  small  intestine;  v,  remnant  of  vitelline 
duct. 


dorsal  part  of  the  cloaca.  It  arises  from  the  ectodermal  (proctodeal)  portion  and 
extends  forward,  dorsal  to  the  rectum  (fig.  236).  In  some  cases  it  degenerates  in 
the  adult. 

The  limits  of  large  and  small  intestine  in  the  mammals  are  usually  marked  by  an 
ileo-colic  valve  and  a  single  caecum,  but  there  are  two  cajca  in  some  edentates, 
while  some  edentates,  bats,  carnivorous  mammals  and  many  whales  lack  either 
caecum  or  valve.     The  caecum  is  larger  in  the  herbivorous  forms  and  frequently 


DIGESTIVE   ORGANS. 


231 


there  is  a  relation  between  the  development  of  caecum  and  stomach.  The  caecum 
becomes  enormous  in  certain  rodents  and  marsupials  (sometimes  longer  than  the 
body)  and  plays  an  important  part  in  digestion,  being  sometimes  lobulated  or 
furnished  with  internal  folds,  those  of  the  rabbits  being  arranged  in  a  spiral  manner. 
In  man  and  the  anthropoids  and  some  other  forms,  as  is  well  known,  the  distal  part 
of  the  caecum  degenerates  to  a  rudiment,  the  vermiform  appendix,  which  tends  to 
become  obliterated  with  increasing  age. 


Fig.  236.  Fig.  237. 

Fig.  236. — Diagrammatic  longitudinal  section  of  the  cloacal  region  of  a  duck  embryo 
at  the  twenty-second  day  of  incubation,  after  Polndyer.  ag,  anal  groove;  c,  cloaca;  cp, 
cloacal  plate;  /,  bursa  Fabricii;  p,  phallus,  with  caecal  duct;  sp,  stercoral  pouch  of  rectum. 

Fig.  237. — Semidiagrammatic  course  of  intestine  of  new-born  deer  Cenms  canadensis, 
after  Weber,     c,  caecum;  d,  duodenum;  co,  colon;  j,  jejunum;  m,  mesentery. 

Both  small  intestine  and  colon  are  at  first  straight,  but  with  growth  they  become 
longer,  involving  convolutions  varying  in  pattern  and  extent  in  different  groups, 
the  patterns  of  the  colon  being  of  some  systematic  value.  The  full  history  has 
been  worked  out  only  for  man,  two  stages  being  represented  in  figure  238.  The 
genus  Hyrax  is  remarkable  for  a  pair  of  caecal  diverticula  arising  from  the  colon 
(fig.  239).  In  the  monotremes  the  rectum  terminates  in  a  cloaca  as  in  the  saurop- 
sida,  and  the  same  condition  occurs  in  the  young  of  all  higher  mammalia.  In  the 
later  stages,  however,  the  urogenital  and  digestive  openings  become  separated  by 
the  formation  of  a  perineal  fold  between  the  two. 


THE  LIVER  (HEPAR). 

The  liver,  the  largest  gland  in  the  body,  has  several  functions.  It 
secretes  the  bile  (gall)  and  forms  several  internal  products  such  as 
glycogen,  urea  and  uric  acid,  of  great  importance  in  the  animal  economy. 


232 


COMPARATIVE  MORPHOLOGY    OF   VERTEBRATES. 


Fig.  238. — Scheme  of  alimentary  canal  and  mesenteries  in  human  embryos,  30  and  50 
mm.  long,  after  Klaatsch.  c,  caecum;  co,  colon;  d^  duodenum;  ^,  kidney;  r,  rectum;  rd^ 
recto-duodenal  ligament;  rl,  recto-lienal  ligament;  rrd,  recto-duodenal  recess;  5,  stomach; 
50,  spleen. 


Fig.  239. — ^Alimentary  canal  of  Hyrax  capensis  after  Flower,     c,  caecum ;  d,  blind  diverticula 
of  colon;  i,  ileum;  r,  rectum;  5,  stomach;  si,  small  intestine. 


DIGESTIVE   ORGANS. 


233 


The  bile  is  passed  to  the  intestine  by  the  bile  duct  (choledochal  or 
hepatic  duct),  but  the  other  products  are  carried  away  by  the  blood 

(internal  secretion). 

The  anlage  of  the  liver  is  a  ventral  diverticulum  from  the  archenteron 
(p.  206),  which  grows  forward  from  its  point  of  origin,  branches  again 
and  again,  the  ultimate  branches  forming  the  glandular  part  of  the 
organ,  the  proximal  parts  of  the  outgrowth  giving  rise  to  the  bile  duct 
(ocasionally  multiple)  which  empties  into  the  intestine.  As  a  result  of 
this  method  of  formation  the  liver  is  to  be  regarded  as  a  compound 
tubular  gland,  the  lumens  of  the  tubules  forming  the  gall  capillaries 
which  eventually  empty  into  the  duct.  This  tubular  condition  is 
readily  recognized  in  the  ichthyopsida,  but  it  is  masked  in  the  amniotes 
and  especially  in  the  mammals,  in  part  by  the  anastomosis  of  the 
tubules,  in  part  by  the  interrelation  of  the  bile  and  blood-vessels. 

With  development  the  liver  grows  cephalad  from  its  point  of  origin, 
but  this  forward  growth  is  limited  by  the  presence  of  the  blood-vessels 
which  develop  the  sinus  venosus  and  the  hepatic  veins  and  also  contrib- 
ute to  the  septum  transversum  (hepatic 
veins — see  circulation),  and  so  its  later 
increase  must  cause  it  to  grow  in  the  op- 
posite direction.  As  it  increases  in  size 
there  is  an  immigration  of  mesenchyme 
between  the  lobules  and  with  these  the 


Fig.  241. 
b,  gall  bladder;  ch,  choledochar  duct; 


Fig.  240. 

Fig.  240. — Diagram  of  two  tj-pes  of  bile  ducts. 
h,  hepatic  ducts;  i,  intestine. 

Fig.  241 . — Liver  and  pancreas  of  American  ostrich  (Rhea)  after  Gegenbaur.     d,  duo- 
denum; rfA,  bile  ducts;  /,  liver;  oe,  oesophagus;  p,  pancreas;  pd,  pancreatic  duct;  s,  stomach. 


blood-vessels  enter.  At  the  same  time  the  liver  grows  away  from  the 
alimentary  canal,  carrying  the  peritoneum  before  it  so  that  it  receives 
an  outer  serous  coat. 

Usually  the  bile  duct  (when  there  are  several  ducts  only  one  is  con- 
cerned) forms  a  lateral  diverticulum,  the  gall  bladder,  which  serves  as 
a  reservoir  for  the  bile.     This  is  usually  placed  on  the  dorsal  side  of  the 


234        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

liver,  but  it  may  be  immersed  in  the  substance  of  the  gland.  In  some 
cases,  even  in  mammals,  the  gall  bladder  may  be  lacking.  When  a 
gall  bladder  is  present,  three  regions  may  be  recognized  in  the  bile 
ducts.  Those  parts  which  lead  from  the  liver  to  the  connexion  with 
the  bladder  are  called  hepatic  ducts ;  these  are  met  by  the  cystic  duct 
leading  from  the  bladder,  and  the  common  duct,  formed  by  the  two  and 
which  empties  into  the  intestine  is  the  choledochal  duct  (fig.  240). 
The  shape  of  the  gland  is  in  part  determined  by  the  shape  of  the  body, 
being  long  in  elongate  species,  sometimes  consisting  of  two  consecutive 
lobes.  Another  modifying  factor  is  the  shape  and  size  of  the  adjacent 
organs,  stomach,  etc.  Usually  the  liver  is  divided  into  right  and  left 
halves,  these  corresponding  to  the  first  division  of  the  anlage,  but  these 
halves  are  hardly  indicated  in  some  of  the  teleosts.  Frequently,  and 
especially  in  mammals,  the  halves  become  subdivided  into  lobes  of 
varying  size,  which  are  arranged  in  various  ways.  The  liver  is  rela- 
tively larger  in  the  ichthyopsida  than  in  the  amniotes,  but  the  cyclo- 
stomes  have  a  small  liver,  that  of  the  myxinoids  being  in  two  parts.  It 
is  larger,  too,  in  the  flesh-eating  than  in  the  herbivorous  species.  The 
blood  supply,  chiefly  through  the  portal  vein  and  to  a  less  extent  by  the 
hepatic  artery  (see  circulation)  is  very  large.  The  color  of  the  gland 
is  very  variable,  especially  in  teleosts,  where  it  may  be  brown,  yellow, 
purple,  green  and  even  vermilion. 

THE  PANCREAS. 

The  second  largest  of  the  digestive  glands,  the  pancreas,  secretes 
digestive  ferments  of  great  strength  (trypsin,  steapsin,  amylopsin), 
which  digest  both  proteids  and  carbohydrates.  In  some  respects  it 
resembles  the  salivary  glands  and  so  compensates  in  part  for  the 
absence  of  them  in  the  lower  vertebrates  (p.  220).  The  pancreas 
arises  by  diverticula  from  the  wall  of  the  intestine  close  to  the  liver. 
There  are  usually  three  of  these  diverticula,  one  dorsal  and  two  ventral, 
the  ventral  soon  uniting  (fig.  242),  but  in  the  sharks  there  is  only  a 
single  dorsal,  diverticulum,  while  in  the  sturgeon  there  are  two  dorsal  and 
two  ventral.  In  a  general  way  these  develop  much  like  theliver,  the  distal 
portions  of  the  divisions  forming  the  glands,  which  are  of  the  acinous 
type;  the  proximal  portions  form  the  ducts.  Of  these  ducts  all  may 
persist;  all  but  one  may  disappear,  while  in  the  lampreys  all  may  be 
lost.     In  many  mammals  two  ducts  persist,  the  ventral  forming  the 


RESPIRATORY    ORGANS.  235 

main  pancreatic  duct  (Wirsung's  duct),  the  dorsal,  the  accessory 
or  Santorini*s  duct.  The  ducts  may  remain  distinct;  they  may  unite 
before  entering  the  intestine  or  one  of  them  may  unite  with  the  bile 
duct. 

For  a  long  time  it  was  supposed  that  a  pancreas  was  lacking  in 
certain  vertebrates  (some  teleosts,  dipnoi,  cyclostomes) ,  but  recent 
studies  have  shown  its  presence  in  many  of  these.     In  the  case  of  some 


ccj> 


Fig.  242. — Diagram  of  developing  pancreas  of  cat,  after  Thyng.  c,  ductus  coledo- 
chus;  d,  duodenum;  dp,  dorsal  pancreas;  ddpy  its  duct;  »,  small  intestine;  s,  stomach;  vp, 
ventral  pancreas. 

teleosts  it  occurs  as  a  slender  tube  in  the  mesentery;  in  the  dipnoi  it  is 
outside  of  the  muscles  in  the  intestinal  wall,  while  in  the  cyclostomes 
it  is  partly  concealed  at  the  insertion  of  the  spiral  valve,  partly  (myxi- 
noids)  in  the  liver.  In  these  forms,  owing  to  the  complete  disappearance 
of  the  duct  it  becomes  a  gland  of  internal  secretion.  The  pancreas 
may  be  elongate,  compact,  or  sometimes  extremely  lobulated.  Usually 
(fig.  241)  it  lies  in  a  loop  of  the  duodenum.  From  certain  peculiarities 
of  structure  the  queston  has  arisen  as  to  whether  two  distinct  structures 
are  included  in  the  pancreas. 

THE  RESPIRATORY  ORGANS. 

The  respiratory  organs  have  for  their  purpose  the  exchange  of  gases 
between  the  blood  and  the  surrounding  medium — water  or  air — 
carbonic  dioxide  being  given  off  and  oxygen  being  absorbed  by  the 
circulating  fluid.  In  order  that  the  exchange  be  readily  effected  it  is 
necessary  that  the  organs  be  richly  vascular,  that  the  walls  between  the 
blood  and  the  surrounding  medium  be  extremely  thin  so  as  to  permit 
rapid  osmosis,  and  that  the  osmotic  surface  be  as  great  as  possible. 
Further,  there  must  be  an  adequate  mechanism  for  passing  the  oxygen- 
containing  medium  over  the  respiratory  surfaces. 


236 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  the  vertebrates  the  organs  of  respiration  are  developed  in  more 
or  Jess  intimate  connection  with  the  cephalic  portion  of  the  digestive 
tract,  just  behind  the  cavity  of  the  mouth.     This  part  of  the  alimentary 

canal,  which  thus  serves  for  the  pass- 
age of  food  and  for  the  performance 
of  respiratory  functions  is  called  the 
pharynx.  The  organs  themselves 
may  take  the  form  of  gills  or  branchiae, 
adapted  for  aquatic  respiration,  or  of 
lungs  (puhnones)  fitted  for  breathing 
air.  In  this  connection  must  be  con- 
sidered the  cases  of  certain  fishes, 
amphibia,  and  turtles  where  respiration 
is  effected  in  part  by  the  skin,  the 
pharyngeal  epithelium,  or  the  diges- 
tive tract.  There  are  also  a  number 
of  other  structures — air  bladder,  thy- 
mus and  thyreoid  glands,  etc.,  which 
are  derived  from  the  pharynx,  though 
they  are  without  respiratory  functions. 

GILLS  OR  BRANCHI^. 

The  typical  gills  or  branchiae  are 
developed  on  the  walls  of  some  of  the 
visceral  clefts  (gill  or  branchial 
clefts)  which  are  formed  in  the  sides 
of  the  pharynx.  These  clefts  arise  as 
paired  pouches  or  grooves  of  the  en- 
toderm of  the  pharynx  (fig.  208). 
They  extend  laterally,  pushing  aside 
the  mesoderm,  until  they  reach  the 
ectoderm,  ectoderm  and  entoderm 
then  fusing  to  a  plate.  This  in  most 
cases  becomes  perforated,  so  that  the 
cavity  of  the  pharynx  is  connected  with  the  exterior  by  a  series  of 
openings  (fig.  243),  the  clefts  developing  in  succession  from  the 
cephalic  end  backward. 

,  ;^  These  visceral  pouches  develop  in  all  vertebrates,  but  in  the  mam- 
mals only  a  few  or  even  none  of  them  break  through  to  the  exterior.    In 


Fig.  243. — Pharyngeal  region  of  a 
young  Acanthias  embryo,  h,  blood- 
vessels; c,  coelomic  cavities  of  gill  arches; 
gy  developing  gills;  gc,  gill  clefts;  h, 
hypophysis;  w,  mouth;  n,  notochord;  o, 
oculomotor  nerve;  oe,  oesophagus;  />, 
peritoneal  cavity;  s,  spiracular  cleft; 
I-III,  first  to  third  head  cavities. 


RESPIRATORY   ORGANS.  237 

the  adult  amniotes  the  pouches  may  completely  disappear  without 
leaving  a  trace,  aside  from  the  Eustachian  tube  (p.  187)  and  the  thymus 
glands  to  be  mentioned  below.  The  number  of  clefts  or  pouches 
varies  between  considerable  limits.  The  largest  number  in  any  true 
vertebrate  (there  are  more  in  Amphioxus  and  the  enteropneusts)  is 
fourteen  pairs  in  some  specimens  of  Bdellostoma.  In  other  cyclo- 
stomes  there  are  seven,  eight  to  seven  in  the  notidanid  sharks,  six  in 
other  elasmobranchs,  five  or  six  in  teleostomes,  amphibia  and  reptiles 
and  five  in  mammals.  In  this  numbering  the  oral  cleft  is  not  included, 
although  there  is  some  evidence  that  the  mouth  has  arisen  by  the 
coalesence  of  a  pair  of  gill  clefts  (p.  206). 

The  serial  repetition  of  the  visceral  clefts  does  not  strictly  correspond  to  the 
other  segmentation  of  the  body,  their  number  and  position  being  at  variance  with 
those  of  the  myotomes.  There  is  a  branchiomerism  or  serial  repetition  of  the 
gill  clefts,  apparently  distinct  from  the  true  metamerism  of  the  head.  The  ap- 
pearance of  these  clefts  or  pouches  and  the  relation  of  aortic  and  branchial  arches 
in  the  amniotes,  where  gills  are  never  developed,  can  best  be  explained  by  the 
assumption  that  these  forms  have  descended  from  branchiate  ancestors. 

Between  each  two  successive  gill  clefts  there  is  an  interbranchial 
septxim,  covered  externally  with  ectoderm,  internally  with  entoderm, 
and  with  an  axis  of  mesoderm,  the  latter  in  the  earlier  stages  carrying 
with  it  a  diverticulum  of  the  coelom  (fig.  243,  c).  Later  blood-vessels 
(aortic  arches)  and  skeletal  elements  (visceral  arches,  p.  63),  are  devel- 
oped in  each  septum,  the  visceral  arches  appearing  on  the  splanchnic 
side  of  the  coelom  and  hence  not  being  comparable  to  ribs  or  girdles. 

In  the  cyclostomes  and  fishes  the  gills  are  developed  from  the  an- 
terior and  posterior  walls  of  the  typical  interbranchial  septa.  They 
were  long  regarded  as  of  entodermal  origin,  but  in  recent  years  con- 
siderable doubt  has  been  thrown  on  this,  at  least  for  the  fishes,  and 
there  is  some  evidence  for  their  ectodermal  origin.  The  matter  cannot 
yet  be  regarded  as  settled.  These  gills  are  either  filamentous  or  la- 
mellate outgrowths  of  epithelium,  each  carrying  a  loop  of  a  blood- 
vessel. Thus  each  typical  cleft  is  bounded  in  front  and  behind  by  gill 
plates  or  filaments  (fig.  246) ,  those  on  a  side  constituting  a  demibranch, 
the  two  demibranchs  of  a  septum  constituting  a  gill,  while  a  cleft  is 
bounded  by  demibranchs  belonging  to  two  gills.  In  the  young 
elasmobranchs  and  in  the  young  of  a  few  teleosts  (before  birth)  the 
gill  filaments  protrude  from  the  clefts  as  long  threads,  but  later  they 
are  withdrawn. 


238 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  the  cyclostomes  and  notidanid  sharks  the  first  cleft  (between  the 
mandibular  and  hyoid  arches)  bears  gills  like  the  rest,  but  elsewhere 
it  dififers.     In  most  elasmobranchs  and  in  a  few  ganoids  {Acipenser^ 


Fig.  244. — Diagram  of  relations  of  oesophagus  and  respiratory  tracts  in  {A)  Myxine  and 
Ammocoetes,  and  {B)  Petromyzon,  b,  bronchus;  oe,  oesophagus;  i,  thyreoid  gland. 

Polyodon,  Polypterus)  it  becomes  reduced  in  size  in  the  adult,  the 
closure  beginning  ventrally  (fig.  136)  so  that  the  persistent  part  of  the 
opening  is  on  the  upper  side  of  the  head.  This  opening  is  called  the 
spiracle.  In  other  vertebrates,  including  the 
chimaeroid  sharks  and  many  true  sharks,  the 
spiracle  is  closed  in  the  adult,  but  in  the  anura  and 
the  amniotes  its  inner  portion  persists  as  the 
Eustachian  tube  and  the  tympanic  cavity  of  the 
ear  (p.  187). 

Usually  the  series  of  gills  begins  wdth  the 
demibranch  on  the  caudal  side  of  the  hyoid  arch, 
while  none  ever  appears  on  the  caudal  side  of  the 
last  cleft.  In  the  teleosts  the  series  of  gills  is  still 
further  reduced,  the  reduction  reaching  its  ex- 
treme in  Amphipnous,  where  there  are  no  demi- 
branchs  on  the  first  and  fourth  branchial  arches 
and  only  one  on  the  second. 

In  the  cyclostomes  the  gill  clefts  occur  at  a  consider- 
able distance  behind  the  mouth,  partly  the  result  of  the 
great  development  of  the  lingual  apparatus.  In  the  larvae 
of  Petromyzon  (Ammocoetes)  the  seven  gill  clefts  are 
nearly  typical,  the  gill  extending  inward  nearly  to  the 
pharyngeal  wall,  each  cleft  having  a  short  efferent  duct 
leading  to  the  exterior,  and  the  oesophagus  beginning  at 
the  hinder  end  of  the  pharynx  (fig.  244,  A).  In  the  meta- 
morphosis to  the  adult  the  oesophagus  grows  forward, 
dorsal   to   the   gill   clefts,   to   the   cephalic   end  of  the 

pharynx,  thus  cutting  ofif  a  ventral  respiratory  tube,  the  so-called  bronchus  (fig. 

244,  B).    At  the  same  time  the  gill-bearing  region  of  each  cleft  becomes  separated 


Fig.  245.  — Gill 
pouches  and  blood-vessels 
of  Myxine,  after  Miiller. 
b,  gill  pouches,  ed,  effer- 
ent ducts;  eo,  external  gill 
opening;  h,  heart;  oe, 
oesophageo-  cutaneous 
duct;  ph,  pharynx. 


RESPIRATORY   ORGANS. 


239 


from  the  bronchus  by  the  development  of  a  short  afferent  duct,  while  the  demi- 
branchs  come  to  lie  in  oval  pouches  (much  as  in  Myxine,  fig.  245),  in  allusion  to 
which  the  cyclostomes  are  sometimes  called  marsipobranchs  (pouched  gills). 

In  the  myxinoids  the  tract  between  the  mouth  opening  and  the  pharynx  is 
more  elongated  arid  the  pharyngeal  region  (fig.  244,  A)  is  not  differentiated  into 
oesophagus  and  bronchus,  as  in  the  adult  lampreys.  In  Myxine  there  are  six  pairs 
of  gills;  in  Bdellosioma  the  number  ranges  from  seven  to  fourteen,  varying  even  on 
the  two  sides  of  our  Pacific  species,  B.  dombeyi.     In  the  petromyzons  and  in 


Fig.  246.- — Diagram  of  gill  clefts  in  {A)  elasmobranchs  and  {B)  teleosts.  A^  and  B\ 
a  single  gill  of  each,  a,  artery;  br,  branchial  ray;  d,  demibranchs;  gc,  gill  chamber;  gr, 
gill  raker;  0,  operculum;  oe,  oesophagus;  00,  opercular  opening;  s,  spiracle;  v,  veins. 

Bdellostoma  the  efferent  ducts  of  the  gill  pouches  open  separately  to  the  exterior;  in 
Myxine  (fig.  245)  they  unite  into  a  common  duct  on  either  side,  the  left  also  receiv- 
ing an  oesophago-cutaneous  duct,  behind  the  last  gill.  This  duct,  which  leads 
from  the  oesophagus  to  the  exterior,  resembles  a  gill  cleft,  but  lacks  gills.  A  similar 
duct  occurs  in  the  same  position  in  Bdellostoma. 

In  the  fishes  there  are  two  types  of  gills  and  associated  structures. 
In  the  elasmobranchs  (the  chimaeroids  excepted)  the  interbranchial 
septum  is  greatly  developed  (fig.  246,  A'),  extending  some  distance 
laj:erally  beyond  the  gill  folds  so  that  the  distal  part  of  the  cleft  forms  an 
excurrent  canal.  This  prolongation  of  the  septum  extends  to  the  ex- 
terior and  then  turns  backward,  thus  protecting  the  delicate  gills  from 


240        FOMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

injury  (fig.  246,  A).  In  other  fishes  the  posterior  margin  of  the  hyoid 
septum  grows  back  as  a  broad  fold  over  the  clefts  behind,  thus  forming 
a  gill  cover  or  operculum  (fig.  246,  B,  0) ,  enclosing  an  extrabran- 
chial  or  atrial  chamber  into  which  all  of  the  clefts  empty  and  which 
in  turn  opens  to  the  exterior  by  a  single  slit  (po)  behind  the  operculum. 
This  opercular  opening  is  usually  broad,  but  it  is  reduced  to  a  circular 
opening  on  either  side  in  a  few  teleosts,  while  in  the  symbranchii  the 
openings  of  the  two  sides  are  united  to  a  single  one  in  the  mid-ventral 
line.  Correlated  with  this  protection  of  the  gills  by  the  operculum  is 
the  reduction  of  the  interbranchial  septum  (fig.  246,  5'),  which  forms 
only  a  slender  bar,  from  which  the  demibranchs  project  far  into  the 
gill  chamber. 


Fig.  247. — Head  of  Chlamydoselache,  after  Garman;/,  opercular  fold. 

Usually  the  two  opercular  folds  are  continuous  beneath  the  pharynx, 
which  points  to  the  beginnings  of  an  operculum  in  the  shark,  Chlamy- 
doselache (fig.  247).  In  the  chimaeroids  the  operculum  is  farther 
developed  and  is  supported  by  cartilaginous  rays.  In  the  teleostomes 
two  parts  may  be  recognized  in  the  operculum,  the  operculum  or  gill 
cover  proper,  supported  by  a  series  of  large  bones  (p.  77),  and  a  more 
ventral  part,  the  branchiostegal  membrane,  which  is  very  flexible 
and  has  a  skeleton  of  slender  (branchiostegal)  rays,  connected  with 
the  hyoid. 

In  the  sea  horses  and  pipe  fishes  (lophobranchs)  the  gills  form  small  rounded 
tufts.  In  the  labyrinthine  fishes  there  is  a  complicated  bony  structure  in  the  bran- 
chial chamber,  covered  by  a  folded  membrane  which  is  used  in  aerial  respiration. 
In  the  young  crossopterygians  (Polypterus,  Calamoichthys)  bipinnate  external  gills 
persist  for  some  time.  In  Amphipnous,  just  referred  to,  a  sac  opening  between 
the  hyoid  and  the  first  branchial  arch  is  developed  on  either  side  of  the  head. 
Its  walls  are  very  vascular  thin  vessels  being  connected  with  both  the  branchial 
arteries  and  the  dorsal  aorta. 

The  gills  are  so  placed  that  there  can  be  an  almost  continuous  stream 
of  water  over  them,  thus  bringing  the  oxygen  needed  by  the  blood.  As 
a  rule,  this  water  is  drawn  in  through  the  mouth  by  the  enlargement  of 
the  oral  cavity,  and  by  its  contraction  is  forced  out  through  the  clefts. 


RESPIRATORY   ORGANS. 


241 


In  the  myxinoids  the  oesophago-cutaneous  duct  is  supposed  to  act  as  the 
incurrent  opening  when  these  animals  burrow  into  fishes.  In  the  lam- 
preys the  water  is  said  to  pass  both  in  and  out  through  the  gill  clefts 
when  these  animals  are  attached  to  some  object.  In  at  least  some  of 
the  elasmobranchs  water  passes  in  through  the  spiracle  which  regularly 
opens  and  closes. 

Many,  if  not  all  of  the  teleosts  have  breathing  valves.  There  are  two  pairs  of 
these,  an  anterior  pair  attached  to  the  margins  of  the  jaws,  which  permit  the  ingress 
of  the  water  but  prevent  its  outflow.  The  other  pair  is  formed  by  the  branchiostegal 
membrane,  which  closes  the  opercular  opening  and  only  allows  -the  water  to  pass 
out.     The  action  of  both  pairs  can  be  easily  seen  from  fig.  248. 


Fig.  248. — Breathing  valves  of  teleosts,  after  Dahlgren.  A,  schematic  figure,  the 
anterior  half  in  the  vertical,  the  posterior  in  the  horizontal  plane;  B,  mouth  of  sunfish 
{Eupomotis) ;  b,  branchiostegal  valve;  mn,  mx,  mandibular  and  maxillary  valves;  v,  oral 
valves. 


In  certain  fishes  with  an  operculum  (Acipenser,  Lepidosteus,  many  teleosts)  a 
gill  is  developed  as  a  series  of  lamellae  on  the  inner  surface  of  the  operculum.  This 
opercular  gill  has  respiratory  functions.  The  pseudobranchs  are  homologous 
with  the  true  gills.  They  are  developed  in  some  elasmobranchs  as  vertical  folds  on 
the  anterior  wall  of  the  spiracular  cleft,  occurring  in  some  cases,  even  where  the 
spiracle  is  closed  externally.  They,  however,  receive  arterial  blood  and  so  cannot 
be  respiratory  in  function.  The  blood,  still  arterial  in  character,  passes  from  them 
to  the  chorioid  coat  of  the  eye  and  in  some  cases  to  the  brain.  From  their  position 
they  must  be  interpreted  as  the  demibranch  of  the  posterior  side  of  the  mandibular 
arch. 

Pseudobranchs  are  common  in  teleosts,  usually  lying  on  the  medial  side  of  the 
hyomandibular  bone.  When  free,  they  are  gill-like  in  appearance,  but  in  some 
species  (fig.  249)  they  are  covered  by  muscles  and  connective  tissue,  when  they 
have  a  blood-red,  glandular  appearance.  Pseudobranchs  also  occur  in  Lepidosteus, 
most  sturgeons  and  Ceratodus;  they  are  lacking  in  Amia  and  Protopterus.  Polyp 
terus  and  Polyodon  have  opercular  gills. 
16 


242 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  the  amphibia  the  gill  clefts  are  formed  in  the  same  way  as  in  the 
fishes,  but  the  first  and  fifth  never  break  through,  and  all  are  usually 
closed  in  the  adult,  the  exceptions  being  in  the  perennibranchs  and 
derotremes  where  from  one  to  three  clefts  remain  open  through  life.  In 
the  urodeles  and  caecilians  there  is  a  reduced  operculum  which  never 
becomes  prominent,  being  merely  a  fold  of  the  integument  in  front  of 
the  gill  area.     In  the  larval  anura  it  is  well  developed,  though  skeletal 


Fig.  249. — Dissection  of  pseudobranchs  (j>5)  and  cephalic  circle  in  pike  {Esox),  after 
Maurer.  cc,  cephalic  circle  e,  vessels  to  eyes;  g,  gills;  n,  vessels  to  palate  and  nose; 
I-IV,  efferent  branchial  arteries. 

Supports  are  lacking,  as  in  all  amphibia.  Before  the  time  of  metamor- 
phosis it  grows  backward  over  the  gills,  gill  clefts,  and  the  anlagen  of 
the  fore  limbs,  and  fuses  with  the  sides  of  the  body  behind  the  latter. 
In  this  way  these  parts  are  enclosed  in  an  extrabranchial  or  atrial 
chamber,  the  chambers  of  the  two  sides  being  in  communication  below. 
During  larval  life  the  branchial  chambers  usually  communicate  with 
the  exterior  by  a  single  excurrent  pore,  usually  on  the  left  side,  but  in 
the  larval  aglossa  right  and  left  excurrent  pores  are  found. 

The  gills  of  the  amphibia  are  certainly  of  ectodermal  origin  (cf.  p. 
237).     First  to  appear  are  the  external  gills,  covered  with  ciliated  epi- 


RESPIRATORY   ORGANS. 


243 


thelium.  Three  pairs  of  these  usually  arise,  before  the  gill  clefts  break 
through,  on  the  outer  surface  of  the  third,  fourth  and  fifth  arches,  and 
they  are  supplied  by  the  corresponding  (aortic)  arches  of  the  blood 
system.  They  are  without  any  skeletal  support  and  are  of  varying  form 
— pectinate,  bipinnate,  dendritic,  etc.  (fig.  250)— and  in  one  species 


Fig.  250. — External  gills  of  young  Amphiuma,  partially  covered  by  opercular  fold. 

of  caecilians,  where  but  a  single  pair  occurs,  they  are  large  leaf-like 
lobes.  When  the  gill  clefts  break  through  there  is  an  ingrowth  of  ecto- 
derm into  each  cleft,  from  which  (except  in  perennibranchs)  gill  fila- 
ments are  developed  on  the  sides  of  the  septa,' so  that  for  a  time  there 
may  be  both  external  and  internal  gills  (fig.  251,  right  side).     In  the 


Fig.  251. — Diagram  of  the  relations  of  external  and  internal  gills  in  the  anuran  tad- 
pole, ifter  Maurer.  ab,  eb,  afferent  and  efferent  branchial  arteries;  A,  heart;  0,  ear  cavity; 
ph\  pharynx;  ra,  radix  aortae. 


perennibranchs  the  external  gills  persist  through  life  (they  are  said  to 
be  absorbed  and  reformed  in  Siren) ,  but  in  other  urodeles  and  in  caecil- 
ians they  are  absorbed  at  the  time  of  metamorphosis.  In  the  anura 
(fig.  251),  as  the  operculum  grows  back  over  the  clefts,  the  external 
gills,  which  are  so  prominent  in  the  earlier  stages,  become  folded  into 
the  extrabranchial  chamber,  where  they  are  gradually  reduced,  while 


244 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


those  belonging  to  the  cleft  become  the  functional  organs,  the  water 
taken  in  through  the  mouth  passing  over  them  in  its  way  to  the  exterior 
via  the  extrabranchial  chamber.  Then,  with  the  completion  of  the 
metamorphosis,  the  lungs  become  functional,  the  gill  clefts  are  closed 
and  the  gills  absorbed,  the  legs  are  developed  and  the  anterior  pair 
released  from  the  extrabranchial  chamber,  the  tail  is  absorbed,  and  the 
tadpole  (larva)  becomes  the  adult. 


Fig.  252. — Cast  of  oropharyngeal  region  of  pig  embryo,  17  mm.  long,  after  Fox.  alf, 
alveo-lingual  fold;  ctm,  cervical  cord  of  thymus;  dp^,  dp'^,  dorsal  apex  of  first  and  second 
pharyngeal  pouches;  dptm,  dorsal  plate  of  thymus;/,  filiform  appendix  of  second  pouch; 
ilr,  lateral  thyreoid;  stt,  sulcus  tubo-tympanicus;  im,  thymus;  vf,  vestibular  fold  of  mouth. 

Little  is  known  of  the  gills  in  the  stegocephals,  but  the  presence  of  well  developed 
branchial  arches  in  the  larvae  of  some  species  (p.  83)  would  imply  the  existence 
of  functional  gills. 

For  some  time  it  was  thought  that  the  fish  gills  were  of  entodermal  origin,  and 
those  of  the  amphibia  were  derived  from  the  ectoderm.  Hence  the  conclusion  was 
that  the  two  had  no  genetic  connexion,  the  gills  of  the  amphibia  being  a  new 
acquisition,  developed  within  the  group  or  arising  from  the  external  gills  of  some 
form  like  Polypterus.  Lately  the  doubts  thrown  upon  the  entodermal  origin  of  the 
gills  of  fishes  (p.  237)  render  it  possible  that  all  vertebrate  gills  are  homologous. 

Gills  are  never  developed  in  the  amniotes,  but  in  the  embryos  the 
paired  visceral  pouches  are  formed  (figs.  208,  252) — five  in  the  saurop- 
sida,  four  in  mammals — in  the  same  way  as  in  the  fish-like  forms. 
Few,  if  any,  of  them  break  through  to  the  exterior,  although  their 
position  is  indicated  by  grooves  on  the  outside  of  the  neck.  The  proc- 
ess of  obliteration  of  these  e^Kternal  grooves  is  interesting.  The  ante- 
rior arches  enlarge  and  slide  back  over  the  posterior,  so  that  at  least 
the  external  branchial  grooves  lie  in  the  wall  of  a  pocket,  the  cervical 
sinus,  on  either  side  of  the  neck  (fig.  253).  Later  a  process  of  the 
anterior  (hyoid)  arch  extends  over  and  closes  the  sinus,  a  process  re- 


RESPIRATORY   ORGANS. 


245 


calling  the  history  in  the  anura.  Internally  the  entodermal  branchial 
pouches,  with  the  exception  of  the  first,  disappear,  but  the  first  persists 
as  the  tympanic  cavity  and  Eustachian  tube  described  in  connexion 
with  the  ear. 


Fig.  253. — Head  of  human  embryo  with  phar3mgeal  floor  removed,  after  Hertwig. 
Cut  surfaces  lined.  Compare  with  fig.  221.  cs,  cervical  sinus;  e,  eye;  h,  hyoid  arch;  hd, 
hypophysial  duct  (Rathke's  pocket);  I,  lung;  Ig,  lacrimal  duct;  w,  naris;  md,  mandible; 
an,  oronasal  groove;  tr,  trachea. 


Pharyngeal  Derivatives. 

Several  structures  arise  in  the  pharyngeal  region — some  developed 
from  gill  clefts,  some  from  other  parts — which,  while  not  respiratory 
in  character,  naturally  come  for  mention  here. 

Among  these  are  the  thymus  glands.  These  arise  from  the  ento- 
dermal epithelium  at  the  dorsal  angle  of  a  varying  number  of  visceral 
clefts  (elasmobranchs,  clefts  2-6  and  possibly  the  spiracle;  teleosts  and 
caecilians,  2-6;  urodeles,  1-5,  i  and  2  degenerating;  anura,  i  and  2, 
the  latter  only  persisting;  amniotes  3  and  4). 

The  organ  which  results  has  varying  positions  and  shapes  in  the 
different  groups.  It  becomes  richly  vascular,  and  by  the  intrusion  of 
connective  tissue,  assumes  an  acinous  form.  In  Myxine  a  number  of 
lobules  behind  the  gill  region  have  been  regarded  as  a  thymus,  but  now 
are  interpreted  as  pronenephric.  In  some  cases  (fishes,  etc.)  the  thymus 
retains  its  primitive  position  dorsal  to  the  gill  clefts  (usually  above  the 


246 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


fourth  in  teleostomes),  and  it  maintains  its  branchiomeric  character  in 
snakes  and  gymnophiones.  It  may  lie  above  and  behind  the  angle  of  the 
jaw  (most  amphibians),  close  to  the  carotid  arteries  (most  sauropsida), 
sometimes  extending  along  the  neck  (crocodiles  and  birds).  In  the 
young  mammals  the  thymus  (sold  in  the  markets  as  'throat  sweet- 
breads'), which  arises  from  a  single  pair  of  clefts,  is  largely  behind  the 
sternum,  extending  forward  along  the  neck.  Later  it  gradually  grows 
smaller,  the  extreme  development  being  reached  in  man  between  the 
fourteenth  and  sixteenth  years,  but  retaining  its  functional  structure 


Fig.  254. — Schemes  of  the  origin  of  several  pharyngeal  derivatives  in  (A)  Rata,  (B) 
anuran  and  (C)  chick,  after  Verdun,  cd,  carotid  gland ;  e,  epithelial  body ;  gr^  gill  remnants ; 
p,  postbranchial  body;  tm,  thymus;  tr,  thyreoid;  7-77,  gill  pouches  or  clefts. 

until  middle  life.  The  function  of  the  thymus  glands  is  as  yet  unknown ; 
though  leucocytes  are  abundant  in  them,  they  are  not  lymphoidal  in 
character. 

Other  structures  arising  in  the  pharynx,  either  from  the  gill  clefts 
or  from  the  pharyngeal  walls,  are  the '  epithelial  bodies,'  post-branchial 
bodies,  suprapericardial  bodies,  gill  remnants,  etc.,  concerning  which 
little  is  known.  The  carotid  glands  of  the  same  region  are  referred  to 
elsewhere. 

The  thyreoid  gland  cannot  be  dismissed  in  such  a  summary 
manner.  This  is  a  ductless  gland  in  the  pharyngeal  region  of  all 
vertebrates,  ventral  to  the  alimentary  tract.  In  the  lower  vertebrates 
it  arises  as  an  unpaired  pocket  in  the  floor  of  the  pharynx  (fig.  254), 
this  retaining  its  connexion  with  the  parent  tube  in  the  ammocoete 
stage  of  the  lamprey  (fig.  190),  but  at  the  time  of  metamorphosis  it 
loses  its  duct  (as  is  early  the  case  in  all  other  vertebrates)  and  eventu- 


RESPIRATORY   ORGANS.  247 

ally  becomes  follicular.  In  most  vertebrates,  the  anlage,  after  separa- 
tion, forms  a  network  of  epithelial  tubes  before  becoming  follicular. 
Usually  it  exhibits  a  marked  bilaterality,  and  in  amphibia  and  birds 
it  becomes  divided  into  two  glands. 

In  the  elasmobranchs  the  thyreoid  lies  between  the  end  of  the  ventral 
aorta  and  the  symphysis  of  the  lower  jaw;  in  teleosts  the  groups  of 
follicles  lie  around  the  ventral  aorta,  extending  out  on  the  anterior  aortic 
arches.  In  the  urodeles  the  gland  lies  just  behind  the  second  arch  and 
in  the  anura  on  the  hinder  margin  of  the  thyreoid  process  of  the  hyoid 
plate.  In  reptiles  it  is  ventral  to  the  trachea  (at  about  its  middle  in 
lizards,  nearer  its  division  in  other  groups),  while  in  the  birds  the  two 
glands  occur  at  the  base  of  the  bronchi.  In  the  mammals  it  is  usually 
near  the  larynx,  and  while  generally  two-lobed,  it  is  here  and  there 
(monotremes,  some  marsupials,  lemurs,  etc.)  paired. 

Like  the  other  ductless  glands,  the  thyreoid  supplies  the  blood  with 
substances  necessary  to  the  well-being  of  the  organism,  in  the  case  of 
mammals  at  least,  an  iodine-containing  albumen.  Degeneration  or 
extirpation  of  the  thyreoid  result  in  cerebral  trouble.  In  the  ancestral 
vertebrate  the  thyreoid  apparently  had  to  do  with  some  part  of  the 
digestive  work,  as  is  shown  by  its  late  connexion  with  the  pharynx 
in  the  ammoccete. 

In  the  pharynx  and  at  the  entrance  of  the  mouth  into  the  pharyn- 
geal cavity  (isthmus  of  the  fauces)  occur  certain  lymphatic  structures 
called  tonsils,  concerning  which  our  knowledge  is  yet  very  deficient. 
One  account  says  they  arise  from  inwandering  epithelial  cells,  the 
other  maintains  that  they  are  formed  from  the  sub-epithelial  meso- 
derm. Two  different  groups  of  organs  are  included  under  this  name, 
the  true  tonsils  at  the  isthmus  of  the  fauces,  and  the  pharyngeal 
tonsils.  The  latter  may  be  represented  by  lymphoid  structures  in  the 
floor  or  roof  of  the  pharynx  of  urodeles  and  anura.  They  are  well 
developed  in  reptiles  and  birds,  occurring  in  the  latter  behind  the 
choanae.  In  mammals  they  are  inconstant  structures.  The  true 
tonsils  of  mammals  lie  one  on  either  side  of  the  isthmus.  Both 
types  of  tonsils  consist  of  an  adenoid  ground  substance  containing 
numerous  lymph  cells,  and  become  follicular  after  birth. 

THE  SWIM  BLADDER. 

While  the  air  or  swim  bladder  (pnenmatocyst)  is  not  respiratory, 
it  is  included  here  from  its  possible  connexion  with  the  lungs.     It 


248 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


occurs  only  in  teleostomes,  and  while  found  in  most  species  (frequently 
absent  from  bottom-feeding  forms — pleuronectids,  etc.),  it  is  lacking 
here  and  there,  even  among  species  classed  as  physostomous  {Lori- 
caria,  etc.).  In  the  young  of  a  few  sharks  {e.g.^  Scyllium)  there  is  a 
pouch  on  the  dorsal  side  of  the  oesophagus  which  suggests  the  possible 
origin  of  the  organ. 

The  swim  bladder  lies  dorsal  to  the  alimentary  tract,  outside  of  the 
peritoneum  (which  frequently  covers  only  its  ventral  surface)  immedi- 


FiG.  255. — Air  bladder  of  Megahps  cyprinoides,  after  de  Beaufort,  a,  anus;  h,  air 
bladder;  d,  pneumatic  dust  leading  from  the  oesophagus;  /,  ligament;  p,  anterior  part  of 
bladder  extending  to  skull. 

ately  below  the  vertebrae  and  excretory  organs  (mesonephroi).  In 
some  instances  it  extends  the  whole  length  of  the  body  cavity  and 
(clupeids)  may  even  send  diverticula  into  the  head.  In  other  species 
it  may  be  much  shorter.  In  development  it  arises  as  a  diverticulum  of 
the  a;limentary  canal  (fig.  209),  and  in  the  ganoids  and  one  group  of 
teleosts  (physostomi)  it  is  connected  with  the  digestive  tract  throughout 


Fig.  256. — Swim-bladders  of  physostomous  fishes;  A,  pickerel  {Esox);  B,  carp  (Cypri- 
nus);  and  C,  eel  {Anguilla)  after  Tracy.  6,  swim-bladder;  ^,  duct;  g,  red  gland;  oe, 
oesophagus. 

life  by  the  pneumatic  duct.  This  usually  empties  into  the  oesophagus, 
but  it  may  connect  with  the  stomach.  In  most  teleosts,  however,  the 
duct  becomes  closed  at  an  early  date  and  the  bladder  loses  its  connex- 
ion with  the  digestive  tract  (physoclisti) . 

The  swim  bladder  is  usually  unpaired  (paired  in  most  ganoids)  and 
may  be  simple  or  divided  into  two  (rarely  three)  connecting  sacs  (fig. 
256).     It  is  usually  regular  in  outline,  but  diverticula  of  all  kinds  are 


RESPIRATORY   ORGANS. 


249 


common,  the  form  being  most  varied  in  the  physoclistous  species.  In- 
ternally the  walls  may  be  smooth  and  the  cavity  simple,  or  it  may  be  sub- 
divided by  septa  (fig.  257),  or,  as  in  Amia  and  Lepidosteus,  it  may  be 
alveolar,  recalling  the  condition  in  the  lungs  of  higher  vertebrates. 
The  walls  sometimes  contain  striated  muscle,  and  in  some  siluroids  and 
cyprinoids  they  are  more  or  less  calcified,  partly  by  the  inclusion  of 
processes  from  the  vertebrae. 


Fig.  257. — \"entral  view  of  opened  air  bladder  and  Weberian  apparatus  of  Macrones, 


combined  from  Bridge  and  Haddon.  a,  atrial  cavity;  ac,  anterior  chamber  of  air  bladder, 
the  arrows  showing  the  connexion  with  the  posterior  chamber;  de,  endoljonph  duct;  s, 
sacculus;  sc,  scaphium;  sk,  sub  vertebral  keel;  tra,  trc,  anterior  and  crescentic  processes  of 
tripos;  M,  utri cuius. 

The  blood  supply  is  arterial,  coming  from  either  the  aorta  or  the 
coeliac  axis,  in  some  instances  dififerent  portions  receiving  blood  from 
both.  In  the  walls  the  arteries  break  up  into  networks  of  minute 
vessels  (*rete  mirabile'),  these  frequently  making  *red  spots'  on  the 
inner  surface.  From  the  retia  the  blood  passes  to  the  body  veins,  (post- 
cardinal,  hepatic  or  vertebral) .  In  the  ganoids  and  phystomous  species, 
especially  those  with  a  wide  pneumatic  duct,  the  gases  contained  in  the 
swim  bladder  may  be  obtained  directly  from  the  air  or  water,  but  in  the 
physoclists  this  is  impossible  and  the  red  spots  may  be  the  place  of  its 


250        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

secretion  and  possibly  of  its  absorption,  the  probability  being  increased 
by  the  greater  abundance  of  the  spots  in  species  with  closed  ducts. 

While  the  pneumatic  duct  usually  connects  with  the  dorsal  side  of  the  alimentary 
canal,  it  enters  the  left  side  in  Eryihrinus,  and  in  the  mid-ventral  line  in  Polypierus 
and  in  Calamoichthys,  In  Polypterus  the  bladder  arises  from  the  ventral  side  and 
there  are  paired  swim  bladders,  the  right  being  the  longer.  The  blood  in  this  genus 
comes  from  the  efferent  branchial  arteries  and  hence  is  arterial. 

The  swim  bladder  is  supposed  to  have  hydrostatic  functions,  aiding 
in  the  recognition  of  differences  of  pressure  due  to  changes  in  depth. 
In  the  clupeids  the  air  bladder  sends  a  diverticulum  into  the  head, 
there  giving  a  branch  to  each  ear.  In  some  physostomes  (siluroids, 
cyprinids,  gymnonoti)  parts  of  the  anterior  vertebrae  are  modified  into  a 
chain  of  bones — the  Weberian  apparatus — adapted  to  convey  dif- 
ferences of  bladder  pressure  to  the  internal  ears.  One  pair  of  bones 
is  connected  with  the  dorsal  wall  of  the  air  bladder,  a  second  with  a 
diverticulum  (sinus  impar)  of  the  internal  ear,  while  others  are  in- 
tercalated between  these  extremes  (fig.  257) .  Changes  in  the  distention 
of  the  bladder  are  thus  conveyed  to  the  inner  jear  and  probably  affect 
the  sense  organs. 

LUNGS  AND  AIR  DUCTS. 

Lungs  arise  as  a  diverticulum  from  the  ventral  side  of  the  pharynx* 
immediately  behind  the  last  gill  pouch.  The  diverticulum  divides 
almost  as  soon  as  outlined  into  right  and  left  halves,  each  the  anlage  of 
the  corresponding  lung.  As  development  proceeds,  the  two  grow  in  a 
caudal  direction  into  the  trunk,  carrying  the  peritoneum  with  them  as 
they  protrude  into  the  ccelom,  so  that  they  eventually  have  an  entodermal 
lining,  derived  from  the  epithelium  of  the  pharynx;  an  outer  serous 
layer  of  peritoneum,  with  mesenchyme  carrying  blood-  and  lymph- 
vessels,  nerve  and  smooth-muscle  fibres  between  the  two.  In  this 
development  two  parts  are  differentiated,  the  lungs,  the  actual  seat  of 
the  exchange  of  gases,  and  the  air  ducts  leading  from  the  pharynx  to 
•them.  The  ducts  may  consist  of  an  anterior  unpaired  portion,  the 
wind-pipe  or  trachea,  connecting  with  the  pharynx,  and  usually  divid- 
ing at  its  lower  or  posterior  end  into  two  tubes,  the  bronchi,  leading  to 
the  two  lungs.  In  most  air-breathing  vertebrates  the  anterior  part  of 
the  trachea  is  specialized  and  forms  a  larynx.  In  addition  to  these 
parts,  the  mechanism  by  which  air  is  drawn  into  and  expelled  from  the 
lungs  forms  a  part  of  the  respiratory  apparatus. 


RESPIRATORY   ORGANS. 


251 


THE  AIR  DUCTS. 


The  opening  from  the  pharynx  into  the  air  ducts  is  known  as  the 
glottis,  usually  an  elongate  slit  capable  of  being  closed  and  opened 
by  appropriate  muscles.  This  is  immediately  succeeded  by  the  ducts, 
which,  except  in  the  dipnoi,  are  more  or  less  differentiated  into  regions 
and  have  skeletal  supports  in  their  walls. 

In  the  dipnoi  the  glottis  is  either  in  the  mid-ventral  line  (Protopterus)  or  a  little 
to  one  side  (Lepidosiren,  Ceratodus)  and  the  air  duct  passes  up  on  the  right  side 
of  the  oesophagus  to  reach  the  lungs  which  are  dorsal  to  the  alimentary  canal.  The 
tube  is  without  skeletal  supports  and  connects  directly  with  both  lungs  without  any 
division  into  bronchi. 

Larynx.^The  beginnings  of  the  larynx  are  seen  in  the  amphibia, 
where  in  the  lower  types  (Necturus)  a  pair  of  cartilages  are  developed  on 
the  sides  of  the  glottis,  in  the  position  of 
a  reduced  visceral  arch,  each  cartilage 
extending  posteriorly  a  short  distance 
along  the  air  ducts.  In  other  genera  of 
urodeles  the  anterior  end  of  each  lateral 
cartilage  separates  from  the  rest  as  an 
arytenoid,  the  first  of  the  laryngeal  carti- 
lages, imbedded  in  the  walls  of  the  glottis. 
The  rest  of  the  lateral  cartilages  may 
remain  entire  (fig.  258)  or  they  may 
separate  into  a  number  of  pieces,  extend- 
ing along  the  lateral  walls  of  the  trachea 
and  bronchi.  Usually  the  anterior  pair 
of  these  pieces  fuse  in  the  mid-ventral  line,        ^i^-    258.— Trachea,    etc.,   of 

.  Amphtuma,       after      Wilder.       a, 

thus    formmg  the  second    (criCOia)    ele-     arytenoid  cartilages;  6*,  fourth  bran- 

ment    of    the    pharyngeal    framework.    LtdeTtj.tyoph^a^gertS 

These   parts   are   moved  by  antagonistic     cle;  tr,  trachea  with  cartilages  in  its 

muscles.     One  set  of  these,  extending  to 

the  persistent  branchial  arches,  serves  as  dilatators  of  the  glottis;  the 
others,  connected  with  the  laryngeal  cartilages  themselves,  constrict 
the  opening.  In  the  anura  the  cricoid  is  converted  into  a  ring,  with 
the  arytenoid  hinged  w^ithin  and  anterior  to  it,  the  whole  larynx  moving 
anteriorly  to  a  position  between  the  hinder  processes  of  the  hyoid  plate. 
Inside  of  the  short  larynx  thus  framed  by  these  cartilages  are  a  pair 
of  folds  of  the  laryngeal  lining,  the  vocal  cords,  extending  parallel  to 


252 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


the  margins  of  the  glottis.  These  may  be  tightened  or  relaxed,  and 
by  their  vibration  of  their  edges  under  influence  of  the  breath  the 
voice  is  produced. 

The  larynx  is  scarcely  more  developed  in  reptiles.  The  cricoid  is  usually  an 
incomplete  ring,  to  which  the  arytenoids  are  attached,  and  the  whole  is  placed  just 
ventral  to  the  median  part  of  the  hyoid,  with  which  it  is  closely  associated  (fig.  259). 
In  several  reptiles  there  is  a  fold  of  the  mucous  membrane  just  in  front  of  the  glottis 
which  is  supposed  to  represent  the  beginnings  of  an  epiglottis  (infra),  while  in 
geckos  and  chameleons  a  pair  of  folds,  running  dorso-ventrally  in  the  larynx, 
serve  as  vocal  cords.  The  larynx  is  also  rudimentary  in  the  birds,  its  place  as  a 
vocal  organ  being  taken  by  the  syrinx  to  be  described  below,  in  connexion  with  the 
trachea.     The  arytenoids  are  frequently  ossified  in  birds. 


Fig.  259.  Fig.  260. 

Fig.  259. — Laryngeal  apparatus  of  Chelone,  after  Goppert.  a,  arytenoid;  b^--,  first  and 
second  branchial  arches;  cr,  cricoid;  d,  dilator  laryngis  muscle;  g,  glottis;  h,  hyoid;  he, 
hyoid  cornua;  sph,  sphincter  laryngis;  tr,  trachea;  cartilage  dotted,  bone  black. 

Fig.  260. — Ventral  and  side  views  of  monotreme  larynx,  after  Gegenbaur.  c,  cri- 
coid; h,  hyoid;  th,  thyreoid;  tr,  trachea. 

In  the  mammals  the  larynx  reaches  its  highest  development.  Its 
framework  is  formed  by  the  arytenoid  and  cricoid  cartilages,  homol- 
ogous with  those  of  the  lower  groups,  and  in  addition,  a  thyreoid 
cartilage  (or  cartilages)  on  the  dorsal  side  anterior  to  the  arytenoids 
and  cricoids.  The  origin  of  the  thyreoid  is  best  seen  in  the  monotremes 
where  the  hyoid  apparatus  enters  into  close  relations  with  the  larynx 
(fig.  260),  while  the  second  and  third  branchial  cartilages  form  two 
plates,  the  lateral  elements  of  the  thyreoid  on  either  side,  the  median 


RESPIRATORY   ORGANS.  253 

element  of  the  hyoid  forming  a  copula.  In  the  higher  mammals  the 
association  of  hyoid  and  larynx  is  not  so  intimate,  even  in  the  embryo, 
but  the  thyreoid  shows  its  double  origin  in  its  development. 

In  the  higher  mammals  the  thyreoid  cartilage  forms  a  half  ring  on 
the  ventral  side  of  the  anterior  end  of  the  larynx,  its  anterior  dorsal 
angles  being  produced  into  cornua  connected  by  ligament  with  the 
hyoid  (fig.  261).  Dorsal  to  the  thyreoid  is  the  glottis  with  the  aryte- 
noids in  its  walls.  Posterior  to  it  is  the  ring-shaped  cricoid,  following 
which  is  the  trachea.     Anterior  to  the  glot'tis  is  a  fold  of  the  mucous 


Fig.  261. — Dorsal  and  side  views  of  larynx  of  opossum,  Didelphys  virginianus  (Prince- 
ton 1739)  cartilages  dotted,  a,  arytenoid;  c,  cricoid;  e,  epiglottis;  g,  glottis;  h,  hyoid; 
t,  trachea;  th,  thyreoid. 

membrane  of  the  pharynx,  the  epiglottis,  supported  by  an  internal  car- 
tilage (possibly  the  fourth  branchial  arch)  which  articulates  with  the 
anterior  margin  of  the  thyreoid.  The  epiglottis  usually  stands  erect, 
leaving  the  glottis  open  for  respiration,  but  during  deglutition  it  folds 
back  over  the  glottis,  thus  preventing  the  entrance  of  food  into  the 
trachea. 

Internally  the  cavity  of  the  larynx  bears  a  vocal  cord  on  either  side. 
These  are  folds  of  the  mucous  membrane,  extending  from  the  thyreoid 
to  the  arytenoids,  and  by  movements  of  these  latter  cartilages  they  can 
be  tightened  or  relaxed,  thus  altering  the  pitch  of  the  note  caused  by 
their  vibration.  Anterior  to  these  cords  is  a  pocket,  the  laryngeal 
ventricle  (sinus  of  Morgagni)  on  either  side,  small  in  most  mammals, 
but  developed  in  the  anthropoid  apes  to  large  vocal  sacs  (in  some 
there  is  a  median  vocal  sac  in  addition),  which  act  as  resonators, 
adding  to  the  strength  of  the  voice. 


254        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

In  the  whales  and  young  marsupials  the  larynx  is  prolonged  so  that  it  projects 
into  the  choana  behind  the  soft  palate.  In  the  whales  (fig.  262)  this  is  an  adaptation 
to  the  manner  of  taking  food  from  the  water  and  breathing  at  the  same  time.  In 
the  young  marsupials  the  milk  is  forced  into  the  mouth  by  the  muscles  of  the  mam- 
m£e  of  the  mother  and  this  arrangement  prevents  strangulation. 

Trachea. — In  the  tetrapoda  the  trachea  is  strengthened  by  the 
formation  of  cartilage  in  its  walls,  the  beginnings  of  which  are  seen 
in  the  urodeles  where  the  fifth  branchial  arch  gives  rise  to  these  ele- 
ments (p.  251).  Their  arrangement  varies  considerably  in  the  urodeles 
and  caecilians,  being  sometimes  scattered  pieces, 
sometimes  regularly  arranged  and  even  united  in 
the  lateral  walls  (fig.  258).  Corresponding  to  the 
posterior  position  of  the  lungs  the  trachea  is  long 
in  these  groups,  but  in  the  anura  it  can  scarcely 
be  said  to  exist,  the  lungs  succeeding  almost 
immediately  to  the  larynx. 

In  the  reptiles  the  trachea  varies  in  length, 
being  shortest  in  lizards  (except   amphisbaenas) , 
longer  in  snakes,  tortoises  and  crocodiles,  divid- 
FiG.  262.— Larynx  of    ing  into  bronchi  at  varying  distances  from  the 
Xiphius  cavirostris  (after   j^ngs.     It  is  frequently  bent  in  turdes.     In  many 

Gegenbaur)     from     side  ®  ^  j  ^  j 

showing  the  prolongation  reptiles  the  cartilage  rings  of  the  trachea  are  in- 
eloir'w  wtLhlr^^c't  complete,  but  in  Sfhenoion,  lizards  and  some 
into  the  choana;  c,  cricoid;    snakes  some  Cartilages  (usually  the  more  anter- 

th,  thyreoid.  .      .    ,  ,  .  ,  1 

lor)  form  complete  rmgs,  the  others  being  com- 
pleted dorsally  by  membrane.  In  snakes  the  successive  rings  are 
often  united,  especially  on  the  sides. 

The  trachea  is  greatly  elongate  in  birds  in  correlation  with  the 
length  of  the  neck  and  the  position  of  the  lungs  within  the  thorax. 
The  rings,  which  are  usually  complete,  are  frequently  ossified.  The 
trachea  is  occasionally  (male  ducks,  etc.)  widened  in  the  middle  and 
in  various  groups  becomes  greatly  convoluted  so  that  its  length  from  the 
glottis  to  the  lungs  exceeds  that  of  the  neck.  In  some  these  convolu- 
tions occur  beneath  the  integument  of  the  thorax;  in  some  between  the 
sternum  and  the  muscles;  and  in  the  cranes  and  swans  within  the 
keel  of  the  sternum. 

The  larynx  is  never  the  organ  of  voice  in  the  birds,  its  place  being 
taken  by  a  somewhat  similar  structure,  the  syrinx,  at  the  division  of 
the  trachea  into  the  bronchi.     The  sound-producing  elements  are 


RESPIRATORY    ORGANS. 


255 


membranes  which  vibrate  by  the  passage  of  air,  as  do  the  vocal 
cords  of  mammals.  Most  common  is  the  broncho-tracheal  syrinx, 
in  which  the  last  rings  of  the  trachea  are  united  to  form  a  reso- 
nating chamber,  the  tympanum,  while  folds  of  membrane,  internal 
and  external  t)mipanic  membranes  (not  to  be  confused  with  the  simi- 
larly named  structure  in  the  ear,  p.  187),  extend  into  the  cavity  from  the 
median  and  lateral  wall  of  each  bronchus.  In  some  cases  there  is  also 
an  internal  skeletal  element  (pessulus)  which  bears  a  semilunar  mem- 
brane on  its  lower  surface.  In  many  birds  this  type  of  syrinx  is 
often  asymmetrical  (fig.  263)  and  is  ex- 
panded into  a  (usually)  bony  resonat- 
ing vesicle.  In  the  tracheal  type  of 
syrinx  the  lateral  port  ons  of  the  last 
tracheal  rings  disappear  and  the  mem- 
brane which  closes  the  gap  forms  the 
vibratile  part.  In  the  bronchial  syrinx 
the  membranes  occur  between  two  suc- 
cessive rings  of  each  bronchus,  each  ring 
being  concave  toward  its  fellow.  By  a 
shortening  of  the  bronchial  wall  these 
membranes  are  forced  as  folds  into  the 
tube.  In  all  types  of  syrinx  there  are 
muscles  attached  to  trachea  and  bronchi, 
which,  by  moving  these  parts,  alter 
the  tension  of  the  folds,  thus  changing 
the  note. 

In  the  mammals  the  trachea  is  elon- 
gate (shortest  in  the  whales  and  sire- 
nians,  dividing  in  the  latter  immedia- 
tely behind  the  cricoid  into  the  two  ^^"^'  ^>''  tympanum. 
bronchi),  and  the  cartilage  rings  are  usually  incomplete  dorsally, 
the  gaps  being  closed  by  membrane.  This  structure  allows  the  tube  to 
remain  open  under  ordinary  conditions  and  yet  allows  it  to  give  when 
food  is  passing  down  the  oesophagus,  just  dorsal  to  it.  In  the  cetacea 
and  sirenia  the  tracheal  cartilages  are  sometimes  spirally  arranged. 

Lungs. 

The  morphology  of  the  lungs  may  be  ujiderstood  by  following  their 
development  in  the  mammals  and  then  describing  their  modifications 


Fig.  263. — Syrinx  of  canvas-back 
duck,  Ayihya,  laid  open  (Princeton 
915).     b,  bronchi;  p,  pessulus;  t,  tra- 


256 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


in  the  various  classes  of  vertebrates.  As  stated  above  the  lungs  arise  as 
a  diverticulum  (fig.  264,  A)  on  the  ventral  side  of  the  pharynx  which 
quickly  divides  into  two  sacs,  the  anlagen  of  the  two  lungs.  These  are 
gradually  pushed  posteriorly  toward  the  body  cavity,  still  retaining 
their  connexion  with  the  pharynx  by  the  air  duct,  and  each  consisting 
of  an  enlarged  terminal  vesicle  connected  by  a  slender  portion  (the 
beginning  of  the  primary  bronchus)  with  the  undivided  tracheal  portion. 
With  continued  growth  each  terminal  vesicle  divides  again  and  again, 
the  result  being  a  number  of  rounded  vesicles  connected  with  the  pri- 
mary bronchi  by  slender  tubes,  the  secondary  bronchi  (fig.  264,  B). 


Fig,  264.  Fig.  265. 

Fig.  264. — Two  stages  in  the  development  of  the  lung  of  the  pig,  ventral  views,  after 
Flint.  A,  pig  5  mm.  long;  B,  18.5  mm.  long,  b,  gill  pouch;  d,  /,  v,  dorsal,  lateral  and 
ventral  bronchi;  oe,  oesophagus;  i,  trachea. 

Fig.  265. — Scheme  of  mammalian  lung  structure,  ad,  alveolar  duct;  b,  bronchus; 
//,  bronchiole;  i,  infundibulum  lined  with  alveoli. 

By  a  continuation  of  this  process  tertiary  and  other  bronchi  are  out- 
lined, and  also  slender  tubes,  the  bronchioles,  to  be  described  later, 
which  connect  the  terminal  vesicles  with  the  ultimate  bronchi.  Next, 
the  inner  wall  of  each  vesicle  becomes  divided  into  small  chambers,  the 
alveoli,  the  whole  vesicle  now  being  known  as  an  infundibulum. 
The  result  of  these  many  divisions  is  an  enormous  amount  of  internal 
respiratory  surface  without  great  increase  in  the  size  of  the  whole 
organ.  It  is  to  be  noticed  that  in  this  subdivision  the  entodermal  li- 
ning takes  the  initiative,  the  outer  (serous)  surface  showing  but  slight 
signs  of  the  internal  modifications. 

Each  infundibulum  has  its  own  duct  which,  when  smooth  internally, 
is  called  a  bronchiole,  when  lined  with  alveoli,  an  alveolar  duct. 


RESPIRATORY   ORGANS. 


257 


The  alveoli  of  infundibulum  and  duct  are  lined  with  squamous 
epithelium,  and  in  the  walls  is  an  extensive  network  of  capillary  blood- 
vessels. The  lining  cells  of  the  bronchioles  are  cubical  and  those  of  the 
bronchi  ciliated  columnar.  There  are  no  skeletal  elements  in  the  bron- 
chioles, but  the  bronchi  have  small  cartilages  in  the  walls,  these  ex- 
hibiting a  tendency  in  the  larger  tubes  to  approximate  the  rings  or 
semi-rings  of  the  trachea. 

In  their  backward  growth  into  the  coelomic  region  the  lungs  either 
insinuate  themselves  dorsal  to  the  lining  of  the  dorsal  side  of  the 
body  cavity  (dipnoi  and  a  few  scattered  forms)  so  that  only  their  ventral 
surface  has  a  serous  coat;  or  they  grow  out  as  free  structures,  covered 
on  all  sides  by  the  coelomic  epithelium,  and  are  bound  to  the  dorsal  wall 
by  a  mesenterial-like  fold  of  varying  extent.  This  outer  coat  of  epithe- 
lium has  received  the  name  of  pleura,  the  term  being  extended  in  the 
case  of  the  mammals  to  include  the 
whole  lining  of  the  pleural  cavity, 
separated  from  the  rest  of  the  coelom 
by  the  diaphragm  (p.  135). 

DIPNOI, — In  Ceratodus  there  is  a  single 
lung  sac;  Protopterus  and  Lepodosiren  have 
paired  lungs,  the  two  being  united  in  front  at 
the  entrance  of  the  air-duct.  In  all  three  the 
inner  surface  is  divided  more  or  less  regu- 
larly into  groups  of  alveoli,  separated  by 
more  prominent  partitions.  The  pulmonary 
arteries  arise  from  the  last  efferent  branchial 
artery  of  either  side,  and  hence  the  blood 
supply,  under  normal  conditions,  is  arterial 
and  the  lungs  cannot  act  as  respiratory 
organs.  In  times  of  drought  (Protopterus) 
or  of  foul  water  (Ceratodus)  the  gills  no  longer  function  and  the  pulmonary  arteries 
bring  venous  blood  to  the  lungs. 


Fig.  266. — Different  types  of  am- 
phibian lungs.  A,  Necturus,  without 
alveoli ;  B,  alveoli  in  the  proximal  por- 
tion; C,  frog,  alveoli  throughout. 


AMPHIBIA. — In  the  lower  urodeles  the  two  lungs  are  elongate  (the 
left  the  longer)  and  are  united  at  their  bases,  true  bronchi  being  absent. 
Internally  they  may  be  entirely  smooth  as  in  Necturus,  or  there  may  be 
alveoli  in  the  basal  portion  (fig.  266),  the  whole  representing  a  terminal 
vesicle  either  connected  directly  with  the  trachea  (A)  or  by  the  interven- 
tion of  an  alveolar  duct  (B).  In  the  caecilians  the  left  lung  is  very  short; 
the  other  elongates,  with  alveoli  developed  throughout.  In  the  frogs 
(fig.  266,  C)  the  two  lungs  are  distinct,  and  their  walls  are  divided  into 
17 


258        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

a  series  of  sacs  or  infundibula  lined  with  alveoli.  The  infundibula 
open  into  a  central  chamber,  which,  since  it  is  ciliated  and  has  numerous 
glands  in  its  walls,  may  be  compared  to  a  bronchiole.  In  the  toads  and 
aglossa  the  alveoli  are  more  extensively  developed  in  correlation  with 
the  more  terrestrial  habits. 

It  has  recently  been  shown  that  a  number  of  terrestrial  urodeles  are  lungless  in 
all  stages  of  development,  and  that  no  traces  of  larynx  or  trachea  occur,  even  after 
the  gills  are  absorbed.  In  these  species  there  is  a  great  development  of  capillaries 
in  the  skin  and  in  the  walls  of  the  mouth  and  pharynx,  the  respiratory  functions 
being  transferred  to  these  parts.  In  the  frogs  the  skin  is  also  respiratory  and  it  is 
largely  supplied  by  the  cutaneous  arteries  which  arise  from  the  same  arch  as  the 
pulmonary  arteries. 

In  the  amphibia  the  air  ducts  enter  the  anterior  end  of  the  lungs, 
but  in  the  amniotes  the  lungs  extend  anteriorly  to  the  entrance  of  the 
bronchi  which  is  on  the  medial  side.  This  change  is  in  part  the  result 
of  the  transfer  of  the  heart  into  the  thorax,  the  position  of  the  pulmonary 
arteries  forcing  the  bronchi  toward  the  centre  of  the  lungs.  In  the 
amniotes,  also,  the  ducts  are  characterized  by  the  presence  of  cartilage 
in  their  walls,  so  that  they  are  true  bronchi.  These  bronchi  may  also 
extend  inside  of  the  lungs,  often  dividing  into  secondary  and  tertiary 
bronchi  inside  them. 

REPTILES. — In  many  reptiles  (snakes,  amphisbaenans,  many 
skinks)  the  lungs  are  asymmetrical  (left  usually  larger  in  snakes,  right  in 
lizards)  and  exceptionally  one  may  be  absent  in  snakes.  The  internal 
structure  shows  considerable  variation.  The  simplest  conditions  are 
found  in  the  snakes  and  in Sphenodon  (fig.  267),  where  the  lungs  consist 
of  a  single  sac  lined  with  infundibula  in  the  basal  portion  (snakes)  or 
throughout  (Sphenodon).  In  the  lizards  (fig.  268)  one  or  more  par- 
titions or  septa  extend  from  the  distal  wall  of  the  lung  nearly  to  the  en- 
trance of  the  bronchus,  thus  dividing  the  lung  into  chambers  lined  with 
alveoli;  while  a  part  of  the  bronchus  may  extend  (main  bronchus, 
fig.  268,  B)  to  the  extremity  of  the  lung.  In  the  chameleons  the  septa 
do  not  reach  the  distal  wall  so  that  the  chambers  communicate  here  as 
well  as  at  the  proximal  side,  the  result  being  that  the  bronchus  enters  a 
cavity,  the  atrium,  which  connects  with  the  chambers  separated  by  the 
septa,  and  these  in  turn  open  into  a  terminal  vesicle,  a  condition  recall- 
ing the  parabronchi  of  the  birds,  soon  to  be  described.  This  resem- 
blance is  heightened  by  the  development  in  these  same  lizards  of  long, 
thin-walled  sacs  from  the  posterior  part  of  the  lung  which  extend  among 


RESPIRATORY   ORGANS. 


259 


the  viscera,  even  into  the  pelvic  region.  These  air  sacs,  which  are 
used  to  inflate  the  body,  foreshadow  the  similarly  named  structures  in 
birds.  In  the  higher  lizards  {Varanus,  fig.  268,  B)  and  the  turtles  and 
crocodiles  there  is  no  atrium,  the  bronchus,  on  entering  the  lung, 
breaking  up  into  several  tubes.  As  these  connect  with  smaller  tubes 
which  lead  to  the  infundibula,  the  whole  lung  has  a  spongy  texture. 


Fig.  267.  Fig.  268. 

Fig.  267. — Lungs  of  Sphenodon,  after  Gegenbaur;  the  left  lung  opened  to  show  the 
alveoli.; 

Fig.  268. — A,  left  lung  of  Iguana;  B,  right  lung  of  Varanus,  after  Meckel,  b,  bronchus 
c,  connection  between  dorsal  and  ventral  chambers;  cb,  chief  bronchus;  d,  dorsal  chamber; 
lb,  lateral  bronchi;  s,  septa;  sb,  secondary  bronchus;  v,  ventral  chamber. 

BIRDS. — In  the  birds  the  lungs  are  closely  united  to  the  ribs  and 
vertebral  column  and  hence  undergo  less  considerable  changes  of 
shape  than  those  of  other  groups.  Each  bronchus  enters  the  meso- 
ventral  surface  of  the  lung,  immediately  expanding  into  a  sac,  the 
atrium  or  ventricle,  and  then  continues  as  a  main  trunk,  the  meso- 
bronchus,  near  the  ventral  side  of  the  organ  (fig.  269).  In  this  course 
it  gives  rise  to  the  secondary  bronchi  (usually  eight  lateral  ectobronchi 
and  from  five  to  six  dorsal  entobronchi)  and  these  in  turn  connect  with 
very  numerous  small  tubes,  the  lung  pipes  or  parabronchi.  These 
run  approximately  parallel  to  each  other  and  connect  with  another 
bronchus  at  the  other  end.  Each  parabronchus  bears  a  number  of 
elongate  diverticula  radiately  arranged  (fig.  270),  these  having  a  nar- 
rower basal  portion  and  being  branched  and  lobulated  distally.     The 


26o         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

PL 


Fig.  269.  Fig.  270. 

Fig.  269. — Diagram  of  structure  of  bird's  lung,  a,  connexions  of  bronchi  with  air 
sacs;  6,  bronchus;  e,  entobronchi;  ec,  ectobronchi;  i,  infundibula;  m,  mesobronchus;  />, 
parabronchi. 

Fig.  270. — A,  lung  pipes  of  bird  from  a  corrosion  preparation;  B,  section  of  lung  pipe 
with  radiating  infundibula,  after  Schulze. 


Fig.  271. — Diagram  of  the  relations  of  the  chief  air  sacs  in  a  bird,  lung  tissue  shaded, 
a,  axillary  sac;  ab,  abdominal  sac;  ai,  anterior  intermediate  sac;  6,  bronchus;  pb,  pre- 
bronchial  sac;  pi,  posterior  intermediate;  sb,  subbronchial  sac;  t,  trachea. 


RESPIRATORY   ORGANS. 


261 


parabronchi  are  to  be  compared  to  bronchioles,  the  diverticula  to 
infundibula. 

The  mesobronchus  and  usually  four  other  bronchi  do  not  stop  at 
the  lung  wall,  but  are  continued  as  thin  walled  vesicles,  the  air  sacs, 


Fig.  272. — Air  sacs  of  pigeon,  after  Bruno  Muller.  c\  c',  intertransverse  canal; 
da^,  da^,  axillary  diverticulum  and  its  ventral  outgrowth;  dCy  diverticulum  costale;  d/a, 
dfp,  divert,  femorale  anterior  et  posterior;  dot,  divert,  oesophago-tracheale;  ds^  div.  sub- 
scapulare;  dst^  div.  stemale;  pc,  preacetabular  canal;  sad,  sas,  saccus  abdominalis  dexter  et 
sinister;  sc,  saccus  cervicalis;  sia,  sip,  saccus  intermedins,  anterior  et  posterior. 

structures  peculiar  to  birds  (and  in  a  slight  extent  to  chameleons)  and 
occurring  in  all  recent  species.  Each  sac  (figs.  271,  272)  has  received 
several  names.  The  sub-bronchial,  anterior  to  the  furcula,  is  usually 
unpaired.     The  cervical,  lateral  to  the  first,  lies  at  the  base  of  the 


262        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

neck,  and  gives  off  a  branch  which  forms  an  axillary  sac  in  the  axillary 
region.  Other  sacs  lie  in  the  abdomen,  lateral  to  the  viscera,  and  are 
called  the  anterior  intermediate,  posterior  intermediate  and 
abdominal,  the  latter  extending  into  the  pelvis.  From  these  air  sacs 
slender  diverticula,  not  shown  in  the  figures,  extend  among  the  viscera 
and  into  certain  of  the  bones.  The  pelvis,  humerus,  coracoid,  sternum 
and  ribs  most  frequently  contain  prolongations  of  the  air  sacs — are 
pneumatic — less  frequently  the  femur,  furcula  and  scapula. 

The  functions  of  the  air  sacs  are  not  certainly  known.  The  fact  that  the  walls 
are  supplied  with  blood  by  branches  from  the  aorta  negatives  the  idea  that  they  are 
respiratory.  It  has  been  suggested  that  they  are  concerned  with  the  maintenance 
of  the  equilibrium  of  the  body  during  flight  and  that  they  also  lessen  the  specific 
gravity  of  the  body.  More  plausible  is  the  view  that  by  the  motion  of  the  parts 
about  them  they  aid  in  the  inspiration  and  expiration  of  air,  especially  during  flight, 
thus  allowing  the  thoracic  framework  to  remain  rigid  as  an  attachment  of  the 
muscles,  and  at  the  same  time  causing  the  air  to  pass  twice  over  the  respiratory 
surfaces  of  the  lungs.  The  bones  of  the  fossil  bird  Archeceopteryx  were  not  pneu- 
matic but  those  of  some  of  the  dinosaurian  reptiles  were. 

MAMMALS. — The  general  structure  of  the  mammalian  lung  was 
outlined  above  (p.  256),  The  external  shape  is  largely  due  to  the 
position  in  the  pleural  cavity,  where  it  has  to  fit  itself  around  the  peri- 
cardium, while  it  is  flattened  or  truncate  behind  as  a  result  of  the 
presence  of  the  diaphragm.  In  a  number  of  mammals  (cetacea,  sirenia, 
horse,  rhinoceros,  Hyrax^  etc.)  both  lungs  are  undivided,  but  usually 
one  or  both  are  subdivided  into  lobes  (the  larger  number  in  the  right 
lung),  there  being  as  many  as  five  or  six  lobes  in  some  species.  In- 
ternally there  is  a  main  bronchus  from  which  dorsal  and  ventral 
secondary  bronchi  arise,  the  ventral  being  the  stronger.  The  bronchi 
are  supported  and  kept  open  by  cartilages,  rings  in  the  larger,  scattered 
pieces  in  the  smaller  trunks.  Frequently  the  bronchi  are  grouped  as 
eparterial  and  hyparterial  (fig.  264),  accordingly  as  they  lie  above 
or  below  the  pulmonary  artery,  but  the  distinction  has  little  morpho- 
logical value.  Eparterial  bronchi  may  be  lacking  or  there  may  be 
one  or  two  in  each  lung. 

The  phylogenetic  history  of  the  lungs  is  uncertain,  one  view  being  that  they 
have  arisen  from  the  air  bladder  of  the  fishes,  the  other  being  that  they  are  modified 
gill  pouches,  which,  instead  of  growing  laterally  and  fusing  with  the  ectoderm, 
have  extended  caudally  and  have  encroached  upon  the  ccelom.  In  favor  of  the 
former  view  are  the  double  condition  of  the  bladder  in  some  ganoids,  with  alveolar 
walls  like  those  of  the  lungs  of  higher  vertebrates,  and  the  peculiarities  of  the  pneu- 


RESPIRATORY   ORGANS.  263 

matic  duct  and  the  blood  supply  in  Polypterus.  On  the  other  hand  the  dorsal 
position  of  the  opening  of  the  duct  into  the  oesophagus  and  the  arterial  supply  from 
the  aorta  in  fishes  are  difficult  to  reconcile  with  the  conditions  obtaining  in  the 
tetrapoda.  Favoring  the  gill-pouch  theory  are  the  following  facts.  The  lungs  are 
paired  outgrowths  from  the  pharynx  immediately  behind  the  kist  gill  cleft;  the 
blood  supply  can  readily  be  derived  from  the  branchiate  condition;  while  the  skeletal 
supports  of  the  larynx  have  the  appearance  of  rudimentary  visceral  arches,  and  the 
muscles  of  the  region  are  modified  from  those  of  the  gill  arches. 

The  mechanisms  by  which  air  is  caused  to  enter  the  lungs  (in- 
spiration) or  is  expelled  from  them  (expiration)  differ  considerably 
in  the  various  classes.  In  the  amphibia  air  is  drawn  into  the  mouth  via 
the  nares  by  depressing  the  floor  of  the  oral  cavity.  Then,  the  nares 
being  closed  by  small  muscles,  the  contraction  of  the  mylohyoid  muscle 
forces  the  air  into  the  lungs.  Expiration  is  affected  in  part  by  the 
elasticity  of  the  lungs,  in  part  by  the  muscles  of  the  body  wall.  In 
most  reptiles  the  position  of  the  ribs  is  altered  by  the  action  of  the 
intercostal  muscles,  thus  altering  the  size  of  the  pleuro-peritoneal 
cavity,  to  accommodate  which  air  is  drawn  into  and  expelled  from  the 
lungs.  It  is  difficult  to  understand  how  inspiration  is  effected  in  the 
chelonia,  but  transverse  muscles  run  ventral  to  the  lungs,  and  these  by 
their  contraction,  expel  the  air.  In  the  birds  the  lungs  are  attached  to 
the  ribs  and  vertebrae,  so  that  any  motion  of  the  latter  necessitates  a 
change  in  shape  and  size  of  the  lungs.  In  addition  the  air  sacs,  as 
noted  above,  may  play  a  part  in  the  movement  of  the  air. 

In  the  mammals  the  ribs  are  hinged  at  an  oblique  angle  to  the  verte- 
bral column,  the  angle  being  changed  accordingly  as  the  intercostal 
muscles  are  contracted  or  relaxed,  and  thus  the  size  of  the  thoracic 
cavity  is  increased  or  dimininshed.  Then  the  diaphragm  (p.  135) 
also  plays  an  important  part  in  this  alteration  in  size.  This  transverse 
muscle  forms  a  complete  partition  between  pleural  and  peritoneal 
cavities,  projecting  into  the  former  like  a  dome  when  relaxed.  When 
it  contracts  it  flattens,  thus  increasing  the  size  of  the  pleural  cavity 
and  drawing  air  in  through  the  trachea.  The  abdominal  muscles 
also  have  their  effect.  Expiration  is  caused  in  part  by  the  action  of  the 
intercostal  and  abdominal  muscles,  in  part  by  the  elastic  tissue  and 
smooth  muscles  in  the  lungs  themselves. 

ACCESSORY  RESPIRATORY  STRUCTURES. 

Allusion  has  already  been  made  to  the  pharyngeal  and  dermal 
respiration  of  the  amphibia  (p.  258).     There  are  several  fishes  in  which 


264        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  hinder  part  of  the  alimentary  tract  is  also  respiratory.  Thus  in 
Cohitis  water  is  drawn  in  and  expelled  from  the  anus,  and  the  posterior 
half  of  the  digestive  canal  is  richly  vascular  and  is  the  seat  of  consider- 
able respiration. 

Before  hatching  or  birth  the  lungs  of  the  amniotes  are  unable  to 
function,  while  a  certain  amount  of  oxygen  is  necessary  for  the  devel- 
opment and  the  carbon  dioxide  formed  must  be  carried  away.  This 
respiratory  function  is  assumed  by  the  allantois.  The  allantois  is 
a  ventral  diverticulum  from  the  hinder  part  of  the  alimentary  canal, 
which  during  foetal  or  embryonic  life,  acquires  a  relatively  enormous 
development.  It  extends  beyond  the  body  limits  and  in  reptiles  and 
birds  comes  into  close  relations  with  the  porous  egg  shell,  while  in  the 
mammals  it  plays  an  important  part  in  the  formation  of  the  placenta. 
In  all  these  the  allantois  is  extremely  vascular,  developing  a  rich  net- 
work of  blood-vessels  close  to  the  shell  (sauropsida  and  monotremes) 
or  to  the  walls  of  the  maternal  uterus,  (mammals)  which  serves  for 
the  rather  limited  exchange  of  gases  necessary  for  the  young.  After 
free  life  begins  the  allantois  is  either  absorbed  (sauropsida)  or  is  lost  with 
the  rest  of  the  placenta  (mammals),  only  the  basal  part  persisting  as  the 
urinary  bladder,  described  in  connection  with  the  urogenital  system. 

ORGANS  OF  CIRCULATION. 

The  functions  of  the  circulation  are  two-fold:  to  carry  food  and 
oxygen  to  the  tissues  and  organs  of  the  body  and  to  remove  the  waste 
from  them.  In  addition  it  has  been  made  probable  that  every  activity 
of  the  body  results  in  the  formation  of  peculiar  substances — activators — 
which  have  fixed  and  definite  effects  upon  the  various  organs.  These 
activators  pass  into  the  blood  and  form  the  stimulus  which  may  cause 
other  organs  or  cells,  remote  from  the  place  where  the  activator  is  formed, 
to  act.  This  subject  is  a  new  one  and  much  may  be  expected  from  it  in 
the  future. 

The  structures  concerned  in  the  circulation  are  two  fluids,  the  blood 
and  the  lymph;  and  the  vessels  (vascular  system)  in  which  the  fluids 
circulate,  certain  parts  of  the  vessels  being  specialized  (hearts)  for  the 
propulsion  of  the  blood  and  lymph.  A  blood  heart  occurs  in  all  verte- 
brates in  connexion  with  the  blood  circulation;  most  vertebrates  have 
lymph  hearts  in  connexion  with  the  lymph  vessels,  but  in  the  higher 
groups  the  flow  of  the  lymph  is  due  to  the  blood  pressure  and  also  to  the 
motion  of  the  parts  through  which  the  lymph  vessels  course. 


CIRCULATORY   ORGANS.  265 

BLOOD  AND  LYMPH. 

The  two  circulating  fluids,  blood  and  lymph,  are  much  alike. 
Each  consists  of  a  fluid  portion,  the  plasma,  in  which  float  numer- 
ous solid  particles,  the  corpuscles.  The  plasma  is  colorless  or 
slightly  yellow  and  can  be  separated  by  clotting  into  a  solid  part, 
fibrin,  and  a  fluid,  the  serum,  which  is,  under  ordinary  circum- 
stances, incapable  of  clotting  again.  The  lymph  plasma  contains 
less  of  the  fibrin-forming  substances  (fibrinogen)  than  does  the  blood 
plasma.  The  composition  of  the  plasma  is  very  complex.  Besides 
water  it  contains  proteids,  extractives,  salts,  and  a  number  of  less- 
known  substances,  internal  secretions,  enzymes,  etc.  The  plasma 
can  also  absorb  a  considerable  amount  of  carbon  dioxide.  It  serves 
to  carry  nourishment  to  the  tissues  and  takes  away  from  them  the 
waste  of  metabolism. 

The  corpuscles  are  of  three  kinds,  erythrocytes,  leucocytes  and 
blood  plates.  Only  the  leucocytes  occur  in  the  lymph  while  the 
blood  contains  all  three. 

The  erythrocytes,  or  red  corpuscles  give  the  blood  its  color. 
They  have  fixed  outlines  and  are  flattened  oval  discs  in  the  non- 
mammals  and  the  camels,  circular  biconcave  discs  in  the  other  mam- 
mals, and  in  all  except  the  mammals  they  are  nucleated  throughout 
their  existence.  They  owe  their  color  to  an  iron-containing  proteid, 
haemoglobin,  which  readily  combines  with  oxygen  and  carbon  dioxide 
and  as  readily  gives  up  these  gases  in  places  where  they  are  scanty. 
This  renders  the  erythrocytes  the  respiratory  elements  of  the  blood. 

It  has  recently  been  stated  that  the  erythrocytes  of  the  mammals  are  hat- 
shaped,  (hollow  cones)  while  inside  the  blood-vessels  and  that  they  assume  the 
biconcave  shape  after  leaving  them.     This  account  has  been  disputed. 

The  size  of  the  erythrocytes  varies  in  different  vertebrates,  being  the  largest 
in  the  amphibia  {Amphiuma)  and  smallest  in  the  vertebrates  (musk  deer).  A 
few  measurements  are  giving  here  in  microns  (o.ooi  mm.).  Where  two  dimen- 
sions are  given  they  are  the  length  and  breadth  of  the  oval  corpuscles.  Musk 
deer,  2.5//;  man,  7.7/z;  hen,  7x12;^;  carp,  9x15/^;  frog,  16X25/X;  Necturus, 
3ix58.5/t;  Amphiuma,  ?xysfi. 

In  the  higher  vertebrates  the  red  corpuscles  arise  by  division  of  giant  cells 
(erythroblasts)  in  the  red  bone  marrow,  but  in  the  young  and  at  times  of  great 
depletion  of  the  blood  new  red  corpuscles  may  be  formed  in  the  spleen  and  the 
liver.     At  first  all  are  nucleated  but  in  the  mammals  the  nucleus  is  soon  lost. 

The  leucocytes  or  white  corpuscles  (divided  accordingly  as  they 


266 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


occur  in  blood  or  lymph  into  leucocytes  and  lymphocytes)  are  very 
variable  in  shape  (amoeboid)  and  may  be  uni-  or  polynucleate.  By 
their  amoeboid  motions  they  are  able  to  pass  through  the  endothelial 
walls  of  the  capillaries  and  to  pass  among  the  cells  of  the  different 
tissues,  hence  they  are  often  called  wandering  cells.  They  have  the 
power  of  ingesting  foreign  bodies  which  renders  them  of  value  in 
combating  pathogenic  organisms;  and  they  also  aid  in  the  absorb tion 
of  fats  and  peptones. 

The  blood  plates  are  very  little  known.  Their  size  is  less  than 
that  of  the  red  corpuscles  and  they  rapidly  degenerate  when  drawn 
from  the  vessels.     They  are  circular  or  elliptical  in  outline. 

THE  BLOOD-VASCULAR  SYSTEM. 

The  blood-vessels  include  the  arteries,  which  carry  the  blood  from 
the  heart  to  all  parts  of  the  body;  the  veins,  which  bring  it  back,  and  the 


Fig.  273. — Embryonic  circulation  of  snapping  turtle,  Chelydra,  showing  relations  of 
allantois,  after  Agassiz  and  Clarke,  a,  right  auricle;  al.  allantois;  av,  allantoic  vessels;  c, 
caudal  vein;  da,  dorsal  aorta;  h,  hypogastric  artery;  ;',  jugular;  /,  liver;  oa,  ov,  omphalo- 
mesenteric artery  and  vein;  pc,  post-cardinal;  sc,  subcardinal  vein;  uv,  umbilical  vein;  w, 
Wolffian  body;  y,  yolk  sac. 

capillaries  which  connect  the  ends  of  the  arteries  and  veins,  for  the 
system  is  closed,  and  there  is  a  complete  circulation. 

Since  all  transfer  of  gases  and  nourishment  takes  place  through  the 
capillaries,  these  vessels  have  extremely  thin  walls,  consisting  of  a 
single  layer  of  squamous  epithelium,  the  so-called  intima.     Usually,  as 


CIRCULATORY  ORGANS.  267 

the  name  implies,  the  capillaries  are  very  small  in  diameter,  but  atten- 
tion has  recently  been  called  to  the  sinusoids,  vessels  with  similar  walls 
but  larger  in  diameter,  which  are  noticeable  in  some  developing  organs, 
especially  the  liver.  Here  also  must  be  mentioned  the  retia  mirabilia, 
places  where  an  artery  or  vein  suddenly  breaks  up  into  a  network  of 
small  vessels  (often  capillary)  which  unite  again,  as  in  the  glomeruli  of 
the  kidney,  to  form  a  vessel  as  large  as  before.  In  the  lymph  nodes 
there  are  similar  networks  of  the  lymph  vessels. 

Arteries  and  veins  (fig.  274)  are  larger  than  the  capillaries  and  they 
have  their  walls  strengthened  outside  of  the  intima  by  layers  of  smooth 


Fig.  274. — Diagram  of  artery  or  vein.     At  the  left  the  intima  alone;  covered  in  the  middle 
by  the  muscularis,  and  at  the  right  with  the  adventitia  added. 

muscle  fibres  (muscle  wall)  and  connective  tissue,  mostly  elastic  (ad- 
ventitial wall).  Since  the  arteries  are  subjected  to  greater  pressure 
than  the  veins  their  walls  are  relatively  much  thicker,  but  in  other  re- 
spects the  two  are  much  alike,  except  that  valves  to  prevent  the  back- 
flow  of  the  blood,  may  occur  in  the  veins,  especially  those  that  are 
vertical  in  the  normal  position  of  the  animal  (legs). 

It  has  been  suggested,  with  much  plausibility,  that  the  main  blood-vessels  are 
the  remnants  of  the  segmentation  cavity,  which  elsewhere  has  been  obliterated  by 
the  increase  of  the  mesoderm.  As  will  be  recalled  (p.  121)  the  mesothelium  grows 
toward  the  middle  line  above  and  below  the  digestive  tract,  thus  tending  to  narrow 
the  segmentation  cavity  in  these  regions  into  two  longitudinal  tubes.  The  epimeral 
part  of  the  mesothelium  divides  into  somites,  and  of  course  the  segmentation  cavity 
extends  between  these,  and  as  these  somites  grow  downward,  these  lateral  exten- 
sions of  the  segmentation  cavity  are  carried  ventrally,  so  that  at  last  they  form  a 
series  of  pairs  of  transverse  vessels  connecting  the  longitudinal  trunks,  thus  forming 
the  vessels  of  the  somatic  wall.  Other  tubes,  connecting  the  dorsal  and  ventral 
trunks,  would  form  between  the  two  walls  of  the  mesentery  and  between  the 
splanchnic  mesoderm  and  the  entoderm,  thus  outlining  the  vessels  of  the  alimentary 
tract. 

Even  more  speculative  is  the  suggestion  that  the  original  circulation  was  lymph- 
oidal  and  that  the  blood  circulation  is  a  specialization  of  a  part  of  this,  the  definitive 
lymph  vessels  being  the  unmodified  part  of  the  primitive  system  of  vessels. 


268        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

An  appreciation  of  this  probable  ancestral  condition  makes  the 
actual  structures  more  easily  understood.  In  development  much  of 
this  phylogenetic  history  has  been  lost,  while  other  parts  have  been 
masked  by  the  development  of  additional  vessels.  Many  vessels, 
which  theoretically  should  arise  as  spaces  between  other  tissues,  are 
actually  formed  as  solid  cords  of  cells,  which  are  later  canalized  and 
converted  into  tubes.  Again,  separate  vessels  of  the  embryo  may  fuse 
during  development  into  a  single  vessel  of  the  adult. 

The  chief  features  of  the  theoretically  primitive  condition  may  be 
summarized  here  (fig.  275).  A  dorsal  tube  carries  the  blood  toward 
the  tail.  From  this  transverse  vessels — right  and  left,  somatic  and 
splanchnic — arise,  which  connect  with  two  ventral  longitudinal  tubes. 


Fig.  275. — Diagram  of  the  primitive  vertebrate  circulation,  a,  anus;  al,  alimentary 
canal;  av,  abdominal  vein;  ca,  cv,  caudal  artery  and  vein;  da,  dorsal  aorta;  h,  heart;  ic, 
intercostal  (somatic)  transverse  vessels;  iv,  intestinal  vessels;  m,  mouth;  si,  subintestinal 
vein;  va,  ventral  aorta. 

one  in  the  wall  of  the  alimentary  tract  and  extending  forward  to  its  junc- 
tion with  the  second  which  runs  in  the  ventral  body  wall,  a  single  tube 
coursing  from  the  point  of  union  to  the  anterior  end  of  the  body. 
In  Amphioxus  various  parts  of  this  system  develop  muscular  walls  and 
act  as  pumping  organs.  In  the  vertebrates,  so  far  as  the  blood  system 
is  concerned,  there  is  a  single  pumping  organ,  the  heart  (the  portal 
heart  of  the  myxinoids  may  be  ignored  in  this  general  statement). 
The  heart  arises  in  the  ventral  tube  beneath  the  pharynx  and  anterior 
to  the  junction  of  the  two  tubes.  It  marks  the  line  of  division  of  the 
transverse  tubes  into  ascending  and  descending,  those  in  front  of  the 
heart  carrying  the  blood  upward  while  those  behind  return  it  to  the 
ventral  vessels  which  carry  it  forward.  The  transverse  vessels  are  not 
continuous,  but  capillaries  intervene  between  their  dorsal  and  ventral 
moieties. 

The  Embryonic  Circulation. 

In  all  vertebrates  a  series  of  blood-vessels  is  laid  down  in  the  early 
stages,  forming  a  framework  around  which  the  rest  of  the  circulation  is 


CIRCULATORY  ORGANS. 


269 


arranged.     Hence  these  parts  are  first  described,  the  additions  and 
modifications  being  taken  up  later. 

The  Heart. 

The  heart,  the  central  organ  for  the  propulsion  of  the  blood,  lies  in  a 
sac,  the  pericardium,  a  part  of  the  coelom,  which  is  ventral  to  the 
pharynx  or  oesophagus  and  is  partially  filled  with  a  serum,  the  per- 


FiG.  276.  Fig.  277. 

Fig.  276. — Diagram  of  the  formation  of  the  heart  tube,  showing  the  descending  meso- 
thelial  plates  from  above,  c,  coelom;  cd,  first  appearance  of  the  Cuvierian  ducts;  A,  grooves 
to  form  heart  and  ventral  aorta;  I,  liver;  m,  mouth;  ma,  mandibular  artery;  om,  omphalo- 
mesenteric veins;  so,  sp,  somatic  and  splanchnic  walls  of  coelom. 

Fig.  277. — Early  stage  of  the  heart;  the  descending  plates  of  fig.  276  have  met,  forming 
the  heart  and  ventral  aorta,  c,  peritoneal  coelom;  ^,  pericardial  coelom;  ppc,  pericardio- 
peritoneal canals;  other  letters  as  in  fig.  276. 

icardial  fluid.  In  the  heart  we  have  to  consider  its  epithelial  lining 
(endocardium),  its  muscular  walls  (myocardium)  and  its  covering 
epithelium  and  connective  tissue  (epicardium). 

The  development  of  the  heart  is  simplest  in  the  vertebrates  with 
relatively  small  yolk.  It  is  more  modified  in  the  elasmobranchs, 
where  the  head  is  early  completed  below,  and  is  most  modified  in  the 
large  yolked  eggs  of  the  sauropsida  and  in  the  mammals  where  the  yolk 
sac  is  large,  though  the  yolk  is  small.  The  following  account  is  based 
upon  the  development  in  the  amphibia: 

From  just  behind  the  point  where  the  first  or  spiracular  gill  cleft 
is  to  form,  backward  to  the  region  just  in  front  of  the  anlage  of  the 


270 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


iver  the  hypomeral  portions  (lateral  plates)  of  the  coelomic  walls  grow 
ventrally  beneath  the  alimentary  canal,  in  much  the  same  way  as 
farther  back  (p.  121).  In  these  descending  plates  splanchnic,  mesen- 
terial and  somatic  walls,  as  well  as  the  coelomic  cavity  can  be  recognized. 
As  they  descend,  cells  which  have  received  the  name  of  vascular 
cells  appear  between  the  coelomic  walls  and  the  entoderm.  The 
origin  of  these  has  been  in  dispute,  but  the  present  evidence  favors 
their  origin  from  the  mesothelium.  Some  of  these  vascular  cells  are 
more  dorsal  and  aid  in  the  formation  of  the  dorsal  blood-vessels,  while 
the  ventral  (fig.  278,  A)  contribute  to  the  heart  and  the  ventral  trunks. 


Fig.  278. — Diagrammatic  cross  sections  of  developing  heart.  Compare  with  figs. 
276  and  277.  In  A  the  descending  mesothelial  plates  have  nearly  met,  a  number  of 
vascular  cells  between  them.  In  B  the  plates  have  met  ventrally,  forming  the  ventral 
mesocardium ;  most  of  vascular  cells  utilized  in  forming  the  endocardium.  In  C  the  plates 
have  met  dorsally,  with  the  resulting  dorsal  mesocardium;  the  ventral  mesocardium  has 
disappeared,  placing  the  two  coelomic  cavities,  now  the  pericardium,  in  communication, 
c,  coelom;  ec,  ectoderm;  en,  entoderm;  end,  endocardium;  m,  edges  of  descending  meso- 
thelium; p,  pericardium;  v,  vascular  cells. 

The  descent  of  the  lateral  plates  continues  until  their  lower  edges 
meet  just  dorsal  to  the  ventral  ectoderm  and  the  ventral  parts  of  the 
mesenterial  regions  of  the  two  sides  fuse  to  a  vertical  plate,  the  ventral 
mesocardiixm  (fig.  278,  B),  above  which  is  a  groove  in  which  the 
ventral  vascular  cells  lie.  Next,  the  edges  of  the  plates  crowd  in  above 
the  groove  and  meet  to  form  a  dorsal  mesocardium,  the  result  being 
that  groove  is  converted  into  a  tube.  The  mesocardia  disappear  early, 
the  ventral  usually  being  lost  before  the  dorsal  is  formed  (fig.  278, 
C).  The  walls  of  the  tube,  which  are  to  form  the  muscular  and  epicar- 
dial  walls  of  the  heart,  are  called  the  myoepicardial  mantle.^  The 
vascular  cells,  which  are  enclosed  in  this  mantle,  gradually  arrange 
themselves  as  a  continuous  sheet,  the  endocardium,  which  lines  the 
future  heart. 

With  the  disappearance  of  the  mesocardia  the  coelomic  spaces  on 
the  two  sides  communicate  with  each  other  so  that  the  myoepicardial 
mantle  lies  free  on  all  sides  in  a  coelomic  sac,  being  bound  to  the  walls 
only  at  the  two  ends.     This  cavity  or  sac  is  the  pericardial  cavity, 

^  The  fact  that  the  heart  muscles  arise  from  this  layer — mesothelial  and  yet  not  myotomic 
— partly  explains  the  differences  between  cardiac  and  other  muscle. 


CIRCULATORY  ORGANS.  27 1 

the  extent  of  which  is  decreased  by  the  fusion  laterally  of  the  somatic 
and  splanchnic  walls  (j5g.  277). 

In  front  of  and  behind  this  tube  the  descending  lateral  plates  are 
kept  from  meeting  in  the  middle  line  by  the  projections  for  the  mouth 
and  liver  (fig.  276).  Vascular  cells,  however,  are  formed  in  these 
regions  and  these  furnish  the  lining  of  tubes  on  either  side,  arising 
in  the  edges  of  the  lateral  plates.  These  tubes  consequently  diverge 
from  the  myoepicardium  in  front  and  behind  and  form  the  first  stages  of 
the  vessels  connected  with  the  heart,  the  anterior  pair  giving  rise  to  the 
mandibular  arteries,  the  posterior  to  the  omphalomesenteric  veins. 
At  about  the  same  time  a  transverse  tube  appears  on  either  side,  which 
connects  with  the  heart  tube,  just  in  front  of  the  division  into  omphalo- 
mesenterics  (fig.  276).  These  transverse  vessels  continue  laterally 
between  the  lateral  plate  and  the  ectoderm,  forming  the  venous  trunks 
known  as  the  ducts  of  Cuvier  (trunci  transversi),  the  other  rela- 
tions of  which  will  be  described  later.  The  ccelom  on  either  side  of 
the  heart  is  restricted  behind  by  the  ridge  formed  by  the  Cuvierian 
ducts  (fig.  277);  with  growth  this  interruption  grows  larger,  the  result 
being  a  transverse  partition,  the  septtim  transverstmi,  which  bounds 
the  pericardial  cavity  behind  and  separates  it  from  the  rest  of  the  ccelom, 
the  peritoneal  cavity.  At  first  this  septum  is  incomplete,  and  in  the 
elasmobranchs  it  never  closes  dorsally  to  the  omphalomesenterics,  but 
leaves  two  openings,  the  pericardio-peritoneal  canals  (fig.  277). 
Elsewhere  the  pericardial  and  peritoneal  cavities  are  entirely  separate 
in  the  adult. 

In  teleosts  and  amniotes,  where  the  eariy  embryo  is  closely  appressed  to  the 
very  large  yolk  sac,  the  development  of  the  heart  is  modified.  At  first  the  pharynx 
is  not  complete  below  but  communicates  ventrally  with  the  yolk.  Hence  the  two 
hypomeres  are  prevented,  for  a  time,  from  meeting  ventrally.  Each,  however,  is 
accompanied  by  its  vascular  cells;  its  edge  becomes  grooved  and  the  grooves  are 
rolled  into  a  pair  of  tubes,  lined  with  endocardium,  so  that  for  a  time  the  anlage  of 
the  heart  consists  of  two  vessels,  each  connected  in  front  and  behind  with  its  own 
mandibular  artery  and  omphalomesenteric  vein,  and  is  surrounded  with  its 
pericardial  sac.  Later  the  two  tubes  approach  and  fuse,  with  the  formation  of 
mesocardia  as  before:  these  latter  soon  disappearing,  leaving  the  whole  much  as 
in  the  small  yolked  forms. 

In  the  early  stages  the  pericardium  is  relatively  large,  but  it  does 
not  keep  pace  with  the  growth  of  the  other  parts,  until  finally  in  the  adult 
it  is  only  large  enough  to  accommodate  the  changes  in  size  and  shape 


272        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

of  the  heart,  due  to  its  alternating  enlargement  (diastole)  and  contrac- 
tion (systole). 

While  the  mesocardia  are  present  the  cardiac  tube  is  a  straight 
canal,  lying  in  the  pericardial  sac  and  connected  with  its  walls  in  front 
and  behind.  With  their  disappearance  the  tube  increases  in  length 
more  rapidly  than  the  pericardium,  the  result  beng  the  flexure  of  the 
tube  on  itself,  something  like  the  letter  00 ,  the  flexures  being  largely 
in  the  vertical  plane.  At  the  middle  point  of  the  flexure  the  tube  re- 
mains small,  forming  the  atrio-ventricular  canal,  but  in  front  of 
and  behind  this  the  walls  become  thickened  and  the  lumen  enlarged. 
The  posterior  and  dorsal  of  the  chambers  thus  formed  becomes  the 
atrium  (auricle),  the  ventral  and  anterior  the  ventricle  of  the  heart. 

The  atrium  is  bounded  posteriorly  by  a  constriction,  behind  which 
the  tube  expands  into  another  chamber,  the  sinus  venosus,  which 
extends  back  to  the  posterior  wall  of  the  pericardium  and  receives  the 
ducts  of  Cuvier  and  the  omphalomesenteric  veins.  The  ventricle,  also, 
does  not  reach  the  anterior  wall  of  the  pericardium,  but  the  anterior 
part  of  the  heart  tube  forms  a  smaller  trunk,  the  truncus  arteriosus, 
while  from  the  pericardium  to  the  mandibular  arteries  is  an  arterial 
vessel,  the  ventral  aorta. 

Muscles,  as  stated  above,  are  developed  in  the  wall  of  the  heart, 
but  to  an  unequal  extent  in  the  different  parts,  being  scanty  in  the 
sinus  venosus,  and  most  abundant  in  the  ventricle.  Folds  or  valves  of 
the  endocardium  appear  in  places  at  an  early  date  and  are  so  arranged 
that  they  permit  the  blood  to  flow  forward  but  prevent  any  backflow. 
In  the  base  of  the  truncus  these  valves  take  the  form  of  pockets  on  the 
walls,  there  being  several  (3-5)  rows  with  several  valves  in  a  row  in  the 
elasmobranchs  (fig.  287,  A)  and  ganoids.  This  valvular  part  of  the 
truncus  is  called  the  conus  arteriosus.  In  other  vertebrates  the  conus 
is  reduced  to  a  single  row  of  valves. 

Valves  also  occur  in  the  atrio-ventricular  canal  (fig.  279)  but  here 
the  pocket-like  condition  is  impossible.  The  folds  extend  from  the 
canal  into  the  ventricle  and  are  prevented  from  folding  back  into  the 
atrium,  under  the  heavy  ventricular  pressure,  by  ligaments — chordae 
tendineae — which  extend  from  the  edges  of  the  valves  to  the  opposite 
wall  of  the  ventricle,  and  are  kept  taut  during  systole  by  short  muscles 
(columnae  carnea)  at  the  base.  Othervalves,  more  simple  in  character, 
occur  around  the  opening  from  the  sinus  into  the  atrium  and,  in  some 
vertebrates,  where  the  hepatic  veins  empty  into  the  sinus. 


CIRCULATORY   ORGANS. 


273 


In  many  fishes  the  conus  arteriosus  is  followed  by  a  strongly  muscu- 
lar region,  the  bulbus  arteriosus  (fig.  287,  B)  which  has  muscles  like 
those  of  the  heart  (p.  125),  while  the  tnmcus  in  front  of  this  has  smooth 
muscles,  like  the  rest  of  the  blood-vessels.  Hence  conus  and  bulbus 
are  to  be  regarded  as  a  part  of  the  heart,  while  the  region  in  front  is  a 
part  of  ventral  aorta  to  be  described  below. 

When  first  formed,  the  heart  lies 
close  behind  the  mandibular  artery  (first 
aortic  arch  to  be  described  below),  but 
as  other  vessels  are  formed  it  is  forced 
farther  back  into  a  position,  in  the  lower 
vertebrates,  ventral  to  and  a  little  behind 
the  pharynx,  but  in  the  adult  tetrapoda 
it  is  carried  back,  as  a  result  of  unequal 
growth  even  into  the  thorax,  the  extreme 
of  migration  being  seen  in  the  giraffe 
and  the  long-necked  birds. 

Although  all  of  the  blood  of  the  body 
passes  through  the  heart  at  short  inter- 
vals, this  is  not  sufficient  for  the  nourish- 
ment of  that  organ.  Therefore  its  mus- 
cles   are    usually    supplied  with   blood 

through  coronary  arteries  which  arise  from  the  aortic  arches  and 
run  back  along  the  truncus  arteriosus  to  reach  the  atrium  and  ventricle. 


Fig.  279. — Diagrammatic  cross 
section  of  heart  showing  atrio- 
ventricular valves;  a,  atriimi;  ct, 
chorda  tendinea;  m,  muscula  pap- 
illosa;  v,  ventricle;  vl,  atrio- ven- 
tricular valves. 


The  Arteries. 


Aorta  and  Aortic  Arches. — The  ventral  aorta  is  the  trunk  in  front 
of  the  pericardium,  extending  from  the  truncus  arteriosus  to  the  mandib- 
ular artery  (first  aortic  arch) .  It  runs,  not  through  a  cavity,  but  be- 
tween muscles  and  through  connective  tissue.  The  mandibular  arter- 
ies continue  dorsally  on  either  side  of  the  pharynx  until  they  reach  its 
dorsal  surface.  With  development,  the  ventral  aorta  elongates  and  at 
the  same  time  other  aortic  arches  arise  between  the  mandibular  arteries 
and  the  pericardium,  these  extending  dorsally  until  they  meet  the  back- 
ward prolongations  of  the  first,  thus  forming  a  pair  of  longitudinal 
tubes,  dorsal  to  the  alimentary  tract,  the  radices  aortae. 

The  number  of  pairs  of  aortic  arches  varies  with  the  number  of  gill 
clefts,  the  vessels  coursing  between  the  clefts.  The  number  of  arches 
18 


274 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


is  greatest  in  the  myxinoids,  where  the  number  of  clefts  varies  (p.  239); 
seven  or  eight  in  the  notidanid  sharks;  and,  as  recent  investigations 
tend  to  show,  probably  six  in  the  embryos  of  all  other  vertebrates.  The 
history  of  these  arches  differs  greatly  in  the  different  classes  (fig.  280), 
there  usually  being  a  reduction  in  number  by  the  more  or  less  complete 


^c 


ic 


m 

ma 


Fig.  280. — Modifications  of  the  aortic  arches  in  different  vertebrates,  after  Boas. 
A,  primitive  scheme;  B,  dipnoan;  C,  urodele;  D,  frog;  E,  snake;  F,  lizard;  G,  bird;  H, 
mammal,  c,  coeliac  artery;  da,  dorsal  aorta;  db,  ductus  Botallii;  ec,  ic,  external  and  internal 
carotids;  p,  pulmonary  artery;  s,  subclavian;  va,  ventral  aorta.  Vessels  carrying  venous 
blood  black,  those  which  disappear,  dotted. 

abortion  of  one  or  more  pairs  as  well  as  a  modification  of  those  that  per- 
sist, accompanying  changes  in  the  respiratory  system. 

With  the  development  of  gills  (ichthyopsida)  each  aortic  arch  be- 
comes divided  into  two  portions,  an  afferent  branchial  artery  convey 
ing  blood  from  the  ventral  aorta  to  the  gills  and  an  efferent  branchial 
artery  (sometimes  called  a  branchial  vein)  carrying  it  from  the  gills 


CIRCULATORY  ORGANS.  275 

to  the  radix  aortae  (fig.  281).  These  two  vessels  parallel  each  other  for 
a  pa  rt  of  their  course  and  are  connected  with  each  other  by  numerous 
capillary  loops  which  run  through  the  gill  filaments.  In  passing 
through  the  gills  the  blood  loses  its  carbon  dioxide  and  takes  up  oxygen, 
and  thus  becomes  converted  from  venous  to  arterial  blood.  In  the  am- 
nio tes  afferent  and  efferent  branchial  arteries  are  never  differentiated,  the 
aortic  arches  being  continuous  from  ventral  aorta  to  the  radices  aortae. 
The  first  of  these  arches  (the  mandibular  arteries)  never  forms 
afferent  and  efferent  portions  since  no  gills  are  ever  developed  in  their 
region.    From  each  half  of  this  arch  an  artery,  the  external  carotid, 


av 


Fig.  281. — Scheme  of  branchial  circulation  in  elasmobranchs.  a,  atrium;  aa,  afferent 
branchial  arteries;  av,  abdominal  vein;  c,  gill  clefts;  cc,  common  carotid;  da,  dorsal  aorta; 
ea,  efferent  branchial  arteries;  hv,  hepatic  vein;  ic,  internal  carotid;  ec,  external  carotid 
artery;  i,  jugular  vein;  /,  liver;  pc,  postcardinal  vein;  sc,  subclavian  vein;  sv^  sinus  venosus; 
tr,  truncus  arteriosus. 

extends  forward  to  supply  the  lower  and  a  part  of  a  upper  jaw,  while  an 
internal  carotid  artery  forms  an  extension  forward  of  each  radix  and 
supplies  the  brain  and  face.  Later  their  relations  are  such  that  the 
carotids  appear  to  arise  from  the  first  of  the  functional  arches. 

The  radices  aortae  of  the  two  sides  meet  and  fuse  behind  the  last 
aortic  arch,  forming  a  single  tube,  the  dorsal  aorta,  w^hich  runs  in  the 
middle  line,  dorsal  to  the  alimentary  tract,  to  the  end  of  the  body.  The 
fusion  may  also  extend  forw^ard  from  the  last  aortic  arch,  involving  the 
whole  of  the  radices. 

From  the  dorsal  aorta  segmental  arteries  extend  laterally  between 
the  somites,  these  forming  the  upper  halves  of  the  transverse  somatic 
vessels  alluded  to  on  page  268.  To  these  the  name  of  intercos- 
tal arteries,  derived  from  human  anatomy,  is  given.  Ventral  to  them 
the  aorta  also  gives  off  other  arteries  (nephridial  arteries)  to  the  excre- 


276        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

tory  organs.  Other  arteries,  arising  from  the  dorsal  aorta,  run  ventrally 
into  the  mesenterial  structures  and  supply  the  alimentary  canal  and 
other  viscera.  Two  pairs  of  these,  the  omphalomesenteric  (omphalo- 
mesaraic)  and  the  hypogastric  arteries,  may  be  mentioned  at  present. 
The  first  of  these  arise  in  the  trunk  region,  pass  on  either  side  of  the 
intestine,  and  finally  empty  on  the  lower  side  of  the  body  into  the  om- 


FiG.  282. — Diagram  of  the  circulation  in  an  early  stage  of  a  small  yolked  vertebrate 
(amphibian),  a,  anus;  ca,  cv,  caudal  artery  and  vein;  da,  dorsal  aorta;  dc,  Cuverian  duct; 
ec,  external  carotid;  h,  heart;  ha,  hypogastric  artery;  i,  intestine;  ic,  internal  carotid;  ij, 
inferior  jugular;  j,  superior  jugular;  /,  liver;  m,  mouth;  oma,  omv,  omphalomesenteric 
artery  and  vein;  pc,  postcardinal  vein;  si,  subintestinal  vein;  1-6,  aortic  arches. 

phalomesenteric  veins,  soon  to  be  described.  The  hypogastric  arteries 
arise  from  the  dorsal  aorta  at  the  junction  of  trunk  and  tail  and  pass  on 
either  side  of  the  intestine,  to  meet  posterior  continuations  of  the 
omphalomesenteric  veins,  here  known  as  the  subintestinal  veins. 
Behind  the  origin  of  the  hypogastric  arteries  the  dorsal  aorta  is  called 
the  caudal  artery  (figs.  275,  282). 

Veins. 

Behind  the  pericardium  the  edges  of  the  descending  lateral  plates 
(p.  270)  are  kept  from  meeting  by  the  anlage  of  the  liver  (figs.  276, 
277) .  The  edges  of  the  plates  become  grooved  just  as  in  front  and  each 
groove  becomes  rolled  into  a  tube,  lined  with  vascular  cells,  so  that  two 
vessels,  the  omphalomesenteric  veins,  extend  backward  from  the  heart, 
around  the  liver,  to  meet  the  omphalomesenteric  arteries  already  de- 
scribed. Behind  the  connection  of  the  omphalomesenteric  arteries  and 
veins  the  pair  of  vessels  continue  back,  ventral  to  the  alimentary  canal 
as  the  subintestinal  veins,  until  just  behind  the  anus  they  fuse  into  a 
median  tube,  the  caudal  vein,  which  extends  the  length  of  the  tail. 

The  two  subintestinal  veins  soon  fuse  to  a  single  median  vessel  (fig. 
283,  B)  save  for  a  loop  around  the  anus  connecting  it  with  the  caudal 
vein.  The  right  omphalomesenteric  vein  disappears  except  for  a  short 
distance  between  the  sinus  venosus  and  the  liver,  leaving  the  left  as  the 


CIRCULATORY  ORGANS. 


277 


trunk  connecting  the  posterior  parts  with  the  heart,  this  passing  along 
the  left  side  of  the  liver  (fig.  283,  B). 

Portal  Circulation. — As  the  liver  develops  from  the  simple  sac  it  is 
at  first,  into  the  compound  tubular  condition  (p.  233),  the  left  omphalo- 
mesenteric breaks  up  into  a  sort  of  rete  mirabile  of  sinusoids,  which 
ramify  among  the  liver  tubules,  finally  connecting  with  both  omphalo- 
mesenterics  on  the  anterior  side  of  the  liver  (fig.  283,  B).  As  the  liver 
increases  in  size  the  network  of  sinusoids  increases  in  complexity, 
supplying  all  of  the  tubules.     For  a  time  the  left  omphalomesenteric 


Fig.  283 . — Three  stages  in  the  development  of  the  hepatic  portal  system.  A ,  primitive; 
B,  liver  tubules  beginning  to  develop,  right  omphalomesenteric  interrupted ;  C,  definitive 
condition,  liver  not  indicated,  dc,  Cuverian  ducts,  hp,  hepatic  portal  vein;  hv,  hepatic 
vein;  /,  liver;  lo,  ro,  left  and  right  omphalomesenteric  veins;  si,  subintestinal  veins;  sv,  sinus 
venosus. 


retains  its  primitive  importance  on  the  side  of  the  liver  and  is  known 
as  the  ductus  venosus  (Arantii),  but  soon  this  preeminence  is  lost 
and  all  blood  coming  from  behind  passes  through  the  network  of  cap- 
illaries in  the  liver  before  it  enters  the  heart  (fig.  283,  C).  Such  a 
capillary  circulation  occurring  in  the  course  of  a  vein  is  known  as  a 
portal  system,  and  this  one  occurring  in  the  liver  is  the  hepatic  portal 
circulation.  It  consists  of  the  vessels  bringing  the  blood  to  the  liver 
(portal  vein) — a  part  of  the  original  omphalomesenteric — the  capil- 
lary vessels  and  the  bases  of  both  omphalomesenterics,  now  known  as 
the  hepatic  veins,  which  convey  the  blood  from  the.  liver  to  the  heart. 

In  eggs  with  a  large  yolk  (elasmobranchs,  sauropsida)  the  presence  of  this 
large  food  supply  exercises  a  modifying  influence  on  these  ventral  veins  (fig.  284). 
From  the  junction  of  the  omphalomesenteric  and  the  subintestinal  veins  a  pair  of 
large  vitelline  veins  run  out  into  the  yolk  sac,  over  the  yolk,  and  play  a  large  part 
in  the  transfer  of  material  to  the  growing  embryo.  The  distal  parts  of  these  veins 
follow  the  margin  of  the  yolk  sac,  forming  a  tube  (sinus  tenninalis)  into  which 


278 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


smaller  veins  empty.  Blood  is  brought  to  the  yolk  by  the  omphalomesenteric 
arteries,  which  are  also  distributed  to  the  yolk  sac,  dividing  up  distally  into  a  net- 
work of  capillaries  connecting  distally  with  the  vitelline  veins.  By  these  the  blood 
is  carried  to  the  liver  and  through  the  portal  circulation  to  the  heart.  In  the  mam- 
mals a  similar  vitelline  circulation  is  developed,  but  as  the  yolk  sac  contains  no 
yolk,  it  is  of  minor  importance. 

In  the  amnio tes  an  outgrowth,  the  allantois  (p.  318),  arises  as  a  diverticulum 
from  the  hinder  end  of  the  alimentary  canal,  increases  in  extent,  growing  downward 
and  carrying  the  ventral  body  wall  before  it.  Branches  of  the  hypogastric  arteries, 
known  as  the  allantoic  arteries,  extend  into  it  and  are  connected  by  capillaries 


Fig.  284. — Diagram  of  embryonic  circulation  in  a  large-yolked  vertebrate;  compare 
with  fig.  282.  aa,  aortic  arches;  al,  allantois;  an,  anus;  ca,  cv,  caudal  artery  and  vein;  da, 
dorsal  aorta;  dc,  Cuverian  duct;  h,  heart;  ha,  hypogastric  (allantoic)  artery;  i,  jugular  vein; 
/,  liver;  oma,  omv,  omphalomesenteric  artery  and  vein;  pc,  postcardinal  vein;  si,  subintes- 
tinal  vein;  st,  sinus  terminalis;  va,  ventral  aorta;  y,  yolk;  ys,  yolk  stalk. 


with  umbilical  veins  which  arise  from  the  subintestinal  vein  behind  the  vitelline 
veins.  There  thus  is  formed  an  allantoic  circulation  which  is  both  respiratory 
and  nutritive  in  character.  In  the  reptiles  both  of  the  umbilical  veins  persist 
through  the  foetal  life  (only  one  shown  in  fig.  273),  but  in  birds  and  mammals  one 
aborts,  leaving  the  other  as  the  efferent  vessel  of  the  allantois.  With  the  end  of 
foetal  life  (at  hatching  or  at  birth)  both  the  vitelline  and  the  allantoic  circulations 
disappear,  leaving  only  inconspicuous  rudiments. 

The  entrance  of  the  Cuverian  ducts  into  the  heart  was  mentioned 
on  page  271.  These  ducts  are  a  pair  of  transverse  vessels  which  enter 
the  sinus  venosus,  one  from  either  side,  and,  together  with  the  hepatic 
veins,  mark  the  posterior  limit  of  the  heart.  Each  develops  outside 
of  the  somatic  wall  of  the  hypomere  and  extends  dorsally  until  it  reaches 
the  level  of  the  top  of  the  coelom  (fig.  282).  In  this  course,  in  the 
fishes,  each  receives  an  inferior  jugular  vein  which  comes  from  the 
head,  bringing  back  blood  from  the  muscles  of  the  lateral  and  ventral 
branchial  regions.     At  its  dorsal  end  each  Cuverian  duct  divides  into 


CIRCULATORY  ORGANS, 


279 


the  two  cardinal  veins,  an  anterior  cardinal  (superior  jugular)  and 
a  postcardinal  vein  (fig.  285),  which  belong  to  the  dorsal  half  of 
the  body.  The  superior  jugular  comes  from  the  head,  dorsal  to  the 
gill  clefts  and  brings  blood  from  the  more  dorsal  regions.  Since  the 
inferior  jugulars  are  found  only  in  fishes  and  salamanders,  the  anterior 
cardinal  is  usually  called  simply  the  jugular  and  that  usage  will  be 
followed  here. 

The  postcardinals  are  closely  related  in  development  to  the  nephric 
system,  and  keep  pace  with  its  development  backward,  so  that  they 
eventually  reach  the  loop  which  the  caudal  and  subintestinal  vein 


Fig.  285. — Developing  anterior  veins  of  Scyllium  embryo,  26  mm.  long;  after  Grosser. 
h  ^-®,  veins  of  the  visceral  arches;  cd,  Cuverian  duct;  h,  vein  of  hyoid  arch;  */,  inferior  jugu- 
lar; w,  vein  of  mandibular  arch;  os,  orbital  sinus;  sv,  segmental  veins;  vca,  vcp,  pre- 'and 
post-cavas;  III-X,  cranial  nerves;  2-8,  spinal  nerves. 

makes  in  passing  around  the  anus.  They  run  just  above  the  dorsal 
side  of  the  coelom  and  dorsal  to  the  nephridial  arteries  (p.  275).  They 
are  preeminently  the  blood-drainage  system  of  the  early  excretory 
organs  and  they  retain  that  function  throughout  life  in  the  lower 
vertebrates. 

Closely  associated  with  the  postcardinals  are  the  subcardinals. 
As  the  mesonephroi  (see  Excretory  Organs)  reach  the  hinder  end  of  the 
ccelom,  the  caudal  vein  loses  its  primitive  connection  with  the  subintesti- 
nal vein  and  becomes  connected  with  a  pair  of  vessels,  the  subcardinal 
veins,  which  develop  between  the  mesonephroi  and  ventral  to  the  nephrid- 
ial arteries  (fig.  286,  B).  The  blood  from  the  tail  now  goes  through 
the  subcardinals  and  from  them  into  the  excretory  organs,  passing 
through  a  system  of  capillaries,  to  be  gathered  again  in  the  postcardinals 


28o 


COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 


and  by  them  to  be  returned  to  the  heart.  Here,  then,  there  is 
another  portal  system  (p.  277),  the  first  renal-portal  system, 
which  may  be  modified  later  as  will  be  described  below. 


Fig.  286. — Scheme  of  development  of  the  principal  veins,  a,  anus;  az,  azygos  major; 
c,  coronary  vein;  ca,  caudal  vein;  cd,  Cuvierian  duct;  ei,  external  iliac;  g,  gonads;  ge^ 
genital  (spermatic,  ovarian)  vein;  h,  hepatic  veins;  ht,  heart;  i,  ischiadic;  j,  jugular;  /»', 
left  innominate;  mn,  mtn,  meso-  and  metanephroi;  om,  omphalomesenterics;  p,  postcava; 
pc,  postcardinal;  pn,  pronephros;  pr,  precava;  r,  renal;  ri,  right  innominate;  s,  subclavian; 
sc,  subcardinal;  si,  subintestinal;  sic,  superior  intercostal. 

In  A  the  early  condition  with  paired  omphalomesenterics  and  subintestinals,  the  post- 
cardinals  extending  back  as  far  as  the  pronephroi.  B,  mesonephroi  developed  and  with 
them  the  subcardinals  and  the  beginning  of  the  postcava;  one  omphalomesenteric  lost  and 
subintestinals  and  caudals  beginning  to  fuse;  the  intestinal  vessels  omitted  in  the  later 
figures.  C,  postcava  has  joined  sinus  and  postcardinals  have  reached  caudals;  D,  amniote, 
appearance  of  metanephroi  (true  kidneys)  with  obsolescence  of  mesonephroi;  the  post- 
cardinals  lose  connexion  with  caudal,  their  place  being  taken  by  the  backward  extension 
of  the  subcardinals;  formation  of  cross  connexions  between  jugulars  and  between  post- 
cardinals  of  the  two  sides.  E,  breaking  up  of  postcardinals  and  disappearance  of  left 
Cuvierian  duct,  the  other  being  called  the  precava. 

Postcaval  elements  crosslined,  subcardinal,  dotted,  other  veins  black. 


The  Definitive  Circulation. 


It  is  impossible  here  to  follow  in  detail  the  development  of  all  parts 
of  the  circulatory  system,  or  even  to  mention  all  of  the  vessels  in  all 
of  the  groups.  All  that  can  be  attempted  is  an  account  of  the  more 
important  parts  and  their  modifications,  with  here  and  there  references 
to  their  history  which  will  render  their  peculiarities  more  intelligible. 
Most  of  the  major  trunks  are  now  known  to  appear  at  first  as  lines 
of  vascular  cells,  similar  to  and  arising  in  the  same  way  as  those  de- 
scribed in  connexion  with  the  heart  (p.  271),  and  it  seems  possible  that 


CIRCULATORY   ORGANS. 


281 


the  intima  of  all  of  the  blood-vessels  is  in  genetic  relations  to  such  lines 
of  cells.  It  should  be  remembered  that  the  vascular  system  is  ex- 
tremely variable,  even  within  the  limits  of  the  species. 


The  Heart. 

The  heart,  as  it  was  left  on  page  273,  was  a  venous  or  branchial 
heart,  in  that  all  of  the  blood  which  enters  it  is  venous  blood  and  is 
all  pumped  directly  to  the  gills  to  lose  its  carbon  dioxide  and  to  take 
up  oxygen,  before  being  distributed  to  the  various  parts  of  the  body. 


^^•'  ->;c  ->i?'  '^P^  yirls-  "nk 


Fig.  287. — DifiFerent  stages  in  the  differentiation  of  the  parts  of  the  heart.  A,  elasmo. 
branch;  B,  teleosts;  C,  amphibia;  D,  lower  reptiles;  E,  alligator;  F,  birds  and  mammals- 
a,  atrium;  ao,  aorta;  b,  bulb  us  arteriosus;  c,  conus;  cd,  Cuvierian  duct;  h,  hepatic  veins;  pa, 
pulmonary  artery;  pc,  pre-  and  postcaval  veins;  pv,  pulmonary  vein;  pa,  pulmonary  artery; 
s,  sinus  venosus;  sa,  septum  atriorum. 

In  its  course  through  the  body  it  passes  but  once  through  the  heart  in 
order  to  make  the  complete  circuit.  Such,  in  general,  is  the  heart  in 
the  cyclostomes  and  fishes  (fig.  287,  A,  B), 

When,  however,  lungs  are  formed  (dipnoi  and  amphibia)  to  share  in 
the  respiratory  processes,  the  heart  begins  to  divide  into  arterial  or 
systemic,  and  venous  or  respiratory  halves.  This  division  is  brought 
about  by  the  formation  of  a  septum  or  partition  in  the  atrium,  partially 
or  completely  dividing  the  chamber,  the  pulmonary  vein  (infra)  open- 
ing into  the  left  half,  which  thus  becomes  arterial,  while  the  sinus, 
with  its  veins,  is  connected  with  the  right  alone  (fig.  287,  C). 

Still  higher  in  the  scale  the  partition  or  septum  extends  through  the 
atrio-ventricular  canal,  dividing  its  valves  into  two  groups  (tricuspid 
valves  on  the  right  side,  mitral  on  the  left)  and  partially  dividing  the 
ventricle  (most  reptiles  fig.  287,  D).     In  the  crocodilia  (fig.  287,  E) 


282  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

the  division  of  the  ventricle  is  completed  by  the  extension  of  the 
septum  to  the  anterior  end,  but  there  is  an  opening  (foramen 
Pannizae)  between  the  two  sides  of  the  aortic  trunk,  so  that  some 
admixture  of  arterial  and  venous  blood  can  occur.  In  the  birds 
and  mammals  (fig.  287,  F)  there  is  complete  internal  separation 
of  the  two  sides  of  the  heart,  though  externally  it  shows  but  slight 
signs  of  the  division.  As  a  result  of  this  division  blood  must  pass  twice 
through  the  heart  (once  through  the  venous,  once  through  the  arterial 
half)  in  order  to  make  a  complete  circuit  of  the  body.  Venous  blood 
enters  the  right  atrium,  passes  to  the  right  ventricle,  by  which  it  is 
forced  to  the  lungs  (puhnonary  or  respiratory  circulation).  Re- 
turning to  the  heart  by  the  pulmonary  veins,  it  passes  through  the  left 
atrium  and  ventricle  and  thence  through  the  systemic  circulation,  by 
which  all  parts  of  the  body  are  supplied.  Details  of  the  modifications 
of  the  heart  in  the  different  classes  of  vertebrates  are  given  at  the  end  of 
this  chapter. 

Aortic  Arches. 

As  was  said  above,  the  typical  number  of  aortic  arches  is  six  pairs, 
this  number  being  but  rarely  exceeded!.  In  all  groups  except  cyclos- 
tomes  and  fishes  they  undergo  considerable  modification,  and  in  the 
fishes  they  are  frequently  more  or  less  reduced  in  correlation  with  the 
reduction  of  the  gills  (p.  238).  The  modifications  may  be  outlined  as 
they  occur  in  the  successive  pairs  of  arches. 

In  many  fishes  and  all  tetrapoda  the  first  arch  on  either  side  dis- 
appears beyond  the  point  where  the  external  carotid  arises,  while, 
correlated  with  the  reduction  of  the  spiracular  gill,  the  second  pair  of 
arches  is  partially  or  completely  lost  in  the  adult.  The  third  pair  is 
always  persistent  and  through  them  flows  the  blood  for  the  internal 
carotids  and,  in  the  fishes,  gymnophiona  and  a  few  urodeles  (fig.  280, 
C)  and  reptiles,  (E)  blood  for  the  radices  aortae  as  well.  In  all  other 
tetrapoda  the  radix  disappears  between  the  third  and  fourth  arches  (fig. 
280,  D)  and  consequently  here  the  third  arch  is  purely  carotid  in  char- 
acter. When  this  occurs  the  portion  of  the  ventral  aorta  between  the 
third  and  fourth  arches  carries  blood  for  the  carotids  alone  and  hence 
forms  a  common  carotid  trunk,  usually  divided  into  right  and  left 
common  carotid  arteries. 

The  fourth  pair  of  arches  are  the  systemic  trunks  in  all  tetrapoda, 


CIRCULATORY  ORGANS. 


283 


carrying  blood  from  the  Ventral  to  the  dorsal  aortae,  while  the  fifth,  re- 
duced in  size,  perform  a  similar  function  in  a  few  urodeles  (fig.  280,  C), 
but  elsewhere  they  entirely  disappear.  The  fourth  arches  show  a  dif- 
ferentiation between  the  two  sides  in  many  reptiles.  That  on  the  left 
side  becomes  separated  from  the  rest  of  the  ventral  aorta  (fig.  280,  £,  F) 
and  has  its  own  tnmk  connecting  with  the  right  side  of  the  partially 
divided  ventricle,  and,  as  will  be  understood  from  the  relations  of  the 
heart  (p.  281),  it  may  carry  a  mixture  of  arterial  and  venous  blood. 
From  the  dorsal  side,  this  blood  of  the  left  fourth  arch  is  largely  dis- 
tributed to  the  digestive  tract,  the  coeliac  axis  arising  from  its  radix, 
while  the  part  connecting  it  with  the  dorsal  aorta  is  reduced  in  size. 
The  right  arch  and  the  carotids  are  connected  with  the  left  side  of  the 


Fig.  288. — Aortic  arches  of  amniotes,  after  Hochstetter.  A,  Varanus;  B,  snake;  C, 
alligator;  D,  bird;  E,  mammal,  b,  basilar  artery;  cc,  common  carotid;  ci,  ce,  internal  and 
external  carotids;  da,  dorsal  aorta;  p,  pulmonary;  s,  subclavian. 

heart  and  hence  are  purely  arterial,  the  arch  forming  the  main  trunk 
connecting  the  heart  with  the  dorsal  aorta.  In  the  birds  (fig.  280,  G) 
the  radix  of  the  left  side  of  the  adult  disappears  distal  to  the  origin  of 
the  subclavian  artery,  so  that  this  arch  supplies  only  the  fore  limb  of 
that  side,  while  the  right  arch  is  purely  aortic  in  character.  In  the 
mammals  (fig.  280,  H)  these  relations  are  exactly  reversed,  the  right 
arch  being  subclavian,  the  left  supplying  the  dorsal  aorta  and  the 
subclavian  of  that  side. 

With  the  development  of  lungs  (dipnoi,  tetrapoda)  a  pair  of  pul- 
monary arteries  are  developed  from  the  sixth  pair  of  arches  on  the 
ventral  side  of  the  pharynx.  These  grow  back  into  the  lungs,  while  the 
rest  of  the  arch,  dorsal  to  their  origin,  becomes  reduced  to  a  small  vessel 
the  ductus  arteriosus  (d.  Botallii)  in  some  urodeles,  and  persists 
occasionally  vestigially  in  higher  vertebrates.  Elsewhere  it  entirely  dis- 
appears.    In  the  dipnoi  and  amphibia,  where  the  ventricle  remains 


284        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

undivided,  the  pulmonary  arteries  are  connected  with  the  same  trunk 
(ventral  aorta)  as  are  the  other  aortic  arches  (fig.  280,  C,  D).  In  the 
amniotes  (£),  F,  G,  H)  with  partial  or  complete  division  of  the  ventricle, 
the  truncus  and  the  ventral  aorta  are  divided  in  such  a  manner  that 
derivatives  of  the  sixth  arch  are  connected  with  the  right  side  of  the  heart, 
while  the  rest  of  the  ventral  aorta,  save  for  the  exception  noted  in  the 
reptiles  above,  receives  its  blood  from  the  left  side  of  the  heart. 

In  connexion  with  the  almost  complete  obliteration  of  the  fifth  arch,  and  in 
most  pulmonate  vertebrates,  the  separation  of  the  sixth  from  the  rest,  it  is  interesting 
to  note  that  in  the  lower  vertebrates  (elasmobranchs)  there  is  already  a  dififerentia- 
tion  of  these  two  arches  from  the  rest  of  the  series  (fig.  281). 

Arteries. 

The  dorsal  aorta  arises  from  the  fusion  of  two  primitive  trunks 
running  approximately  parallel  to  the  notochord,  and  extends  as  a  me- 
dian vessel,  usually  lying  just  dorsal  to  the  origin  of  the  mesentery, 
from  the  point  of  union  of  the  radices  back  nearly  to  the  posterior  end 
of  the  body. 

In  human  anatomy  the  different  parts  of  the  aortic  vessels  have  names  different 
from  those  adopted  here.  The  persistent  portion  of  the  ventral  aorta  is  called  the 
ascending  aorta,  the  persistent  fourth  arch  is  the  arch  of  the  aorta,  and  the 
adjacent  part  of  the  dorsal  aorta  is  the  descending  aorta.  The  rest  of  the  dorsal 
aorta  is  divided  into  the  thoracic  and  abdominal  aortae,  accordingly  as  they  lie 
in  the  regions  of  the  corresponding  cavities.  These  terms  are  inapplicable  in 
comparative  anatomy. 

The  arteries  arising  from  the  dorsal  aorta  may  be  grouped  under  the 
two  categories,  visceral  and  somatic  (p.  268).  To  the  former  belong 
the  vessels  running  through  the  mesenterial-like  structures  (mesen- 
teries, omenta,  mesorchium,  etc.)  to  supply  the  digestive  tract  and  the 
excretory  and  reproductive  organs.  In  the  primitive  condition  those 
going  to  the  alimentary  canal  are  numerous  but  they  do  not  show  a  meta- 
meric  character.  In  the  majority  of  vertebrates  they  become  united 
into  a  smaller  number  of  main  trunks  from  which  branches  go  to  the 
various  regions.  The  principal  of  these  trunks  are  the  following: 
There  is  usually  present  a  coeliac  artery,  arising  from  the  radix  or 
from  the  dorsal  aorta  near  it,  and  dividing  in  the  mesogaster  into 
gastric,  splenic  and  hepatic  arteries,  distributed  to  stomach,  spleen 
and  liver.     The  superior  mesenteric  artery  is  connected  in  develop- 


CIRCULATORY  ORGANS. 


285 


ment  with  the  omphalomesenteric  arteries  (p.  276)  and  goes  to  the  ante- 
rior part  of  the  intestine;  while  frequently  an  inferior  mesenteric  artery- 
is  distributed  to  the  posterior  part  of  the  digestive  tract.  The  superior 
mesenteric  may  fuse  with  the  cceliac  to  form  the  coeliac  axis  while  not 
infrequently  other  mesenteric  arteries  may  be  developed. 

The  hypogastric  arteries,  already  mentioned,  need  further  notice. 
These  primitively  connect  the  dorsal  aorta  with  the  subintestinal  vein 
in  the  neighborhood  of  the  anus,  and  later  give  off  vessels  to  the  region 
of  the  rectum.  When,  as  in  all  classes,  from  the  amphibia  upward,  a 
urinary  bladder  is  developed  from  the  rectal   (cloacal)  region,  the 


Fig,  289. — Diagram  of  vertebrate  circulation  based  on  a  urodele.  Arteries  cross- 
lined;  veins  black  except  the  pulmonary  vein,  white,  av,  abdominal  vein;  a,  ccEliac  arter}'; 
ca,  cv,  caudal  artery  and  vein;  d,  dorsal  aorta;  ec,  external  carotid;  g,  gonad;  h,  hepatic 
vein;  ha,  hepatic  artery;  hy,  hjqpogastric  artery;  ic,  internal  carotid;  il,  iliac  artery  and  vein; 
;,  jugular;  Iv,  Uver;  m,  mv,  mesenteric  artery  and  vein;  pa,  pulmonary  artery;  pcd,  post- 
cardinal;  pcv,  postcava;  pv,  hepatic  portal  vein;  r,  rectal  artery;  ra,  renal  advehent  vein; 
sc,  subclavian  artery  and  vein. 


hypogastrics  form  its  blood  supply,  these  vessels  being  the  vesical 
arteries.  In  the  amnio tes  the  distal  end  of  the  anlage  of  the  bladder 
forms  a  foetal  structure  known  as  the  allantois,  described  in  another 
section  (p.  318),  and  parts  of  the  vesical  arteries  are  carried  out  as 
allantoic  arteries  (fig.  273),  into  the  new  formation.  Since  these 
pass  through  the  umbilicus,  they  are  also  known  as  the  umbilical 
arteries.  Later,  when  the  umbilicus  disappears,  the  allantoic  arteries 
are  lost  and  only  the  rectal  and  vesical  arteries  remain  of  the  hypo- 
gastric trunks. 

The  arteries  going  to  the  excretory  and  reproductiye  organs  are 
paired  and,  in  the  more  primitive  vertebrates  show  a  marked  metamer- 
ism. They  are  best  described  in  details  along  with  the  urogenital 
structures  in  a  subsequent  section.  It  may  be  mentioned  here  that  the 
metamerism  is  well  shown  in  the  nephridial  or  renal  arteries  going  to 


286 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


the  pro-  and  mesonephroi,  while  there  is  usually  but  a  single  pair  of 
renal  arteries  to  supply  the  metanephroi  (true  kidneys)  of  the  amni- 
otes.  The  arteries  to  the  gonads  may  be  included  under  the  single 
head  of  genital  arteries,  though  they  are  usually  subdivided  into  the 
spermatic  and  ovarian  arteries  according  to  the  sex.  Like  the  neph- 
ridial,  the  genital  arteries  are 
more  numerous  in  the  lower 
and  are  reduced  in  number  in 
the  higher  forms. 

The  somatic  arteries  are 
more  numerous  and  are  meta- 
merically     arranged.       In     the 


^a  cjcr 


Fig.  290.  Fig.  291. 

Fig.  290. — Diagram  of  early  relations  of  vertebral  arteries  in  an  amniote.  av,  vertebral 
artery;  da,  dorsal  aorta;  ec,  ic,  external  and  internal  carotids;  pa,  pulmonary  artery;  ra, 
radix  aortae;  sa,  subclavian. 

Fig.  291. — A,  side  view  of  developing  anterior  arteries  oiLacerta,  after  van  Bemmeln; 
the  vertebral  artery  not  developed  behind;  B,  ventral  view  of  the  relations  of  the  arteries  at 
the  base  of  the  vertebrate  brain,  av,  vertebral  artery;  h,  basilar  artery;  cw,  circle  of  Willis; 
da,  dorsal  aorta;  ec,  ic,  external  and  internal  carotids;  pa,  pulmonary  artery;  ra,  radix 
aortae;  sa,  segmental  arteries;  sc,  subclavian;  2-6,  aortic  arches. 

early  stages  they  are  given  off  in  pairs  from  the  radices  and  the 
dorsal  aorta,  an  artery  on  either  side,  extending  laterally  between 
each  two  successive  myotomes  (fig.  275).  Many  of  these  remain 
in  a  slightly  modified  condition  and  are  called  intercostal  arteries 
(including  lumbar  and  sacral  arteries,  etc.,  according  to  position). 
These  usually  become  connected  on  either  side  (fig.  290),  near  their 


CIRCULATORY  ORGANS. 


287 


origin,  by  a  longitudinal  vessel,  the  vertebral  artery,  which,  in  the 
higher  vertebrates,  runs  through  the  vertebraterial  canal  (p.  54)  of 
the  vertebrae. 

In  the  region  of  the  aortic  roots,  after  the  formation  of  the  vertebral 
artery,  all  of  the  segmental  arteries  except  the  last  of  the  series  lose 
their  connexion  with  the  radix  and 
henceforth  are  supplied  by  way  of  the 
posterior  segmental  and  the  vertebral 
(fig.  291).  Anteriorly  the  vertebral 
arteries  pass  to  the  ventral  side  of  the 
spinal  cord  (or  medulla  oblongata) 
dividing  there  into  two  branches,  one 
of  which,  joining  its  fellow  of  the 
opposite  side,  runs  back  beneath  the 
spinal  cord  as  a  spinal  artery,  while 
the  anterior  branches  unite  in  the  same 
way  to  form  a  basilar  artery,  running 
forward  beneath  the  medulla  (fig.  291, 
B).  At  the  point  just  behind  the 
hypophysis  the  basilar  divides,  one-half  passing  on  either  side  of 
that  structure  and  receiving  the  internal  carotid  of  that  side.  The 
trunks  thus  formed  unite  in  front  in  the  region  of  the  optic  chiasma. 
There  is  thus  formed  an  arterial  ring,  the  circle  of  Willis,  round 
the  hypophysis. 


Fig.  292. — Diagram  of  origin  of 
blood  supply  of  vertebrate  appendage. 
V,  abdominal  vein;  da,  dorsal  aorta; 
si,  subintestinal  vein;  so,  somatic  (seg- 
mental) vascular  arch. 


P'iG.  293 . — Three  stages  in  the  development  of  the  arteries  of  the  forelimb  of  the  white 
mouse,  after  Goppert.  A,  8  days;  B,  9  days;  C,  10  days;  a,  aorta;  b,  brachial  plexus. 
•(The  vessels  are  extremely  variable,  not  agreeing  even  on  the  two  sides  of  a  single 
individual.) 


As  the  limbs  grow  out,  segmental  arteries,  corresponding  in  number 
to  the  somites  concerned  in  the  appendages,  grow  out  into  the  member. 
Distally  these  arteries  become  connected  with  each  other  and  with  the 
veins  of  the  limb  by  a  network  of  small  vessels.     By  enlargement  of 


288        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

parts  of  these  main  trunks  and  of  the  connecting  network,  and  the 
partial  or  complete  atrophy  of  other  portions  the  definitive  circulation 
of  the  limb  is  established.  This  explains  the  numerous  variations  in 
the  blood  supply  of  the  limbs,  both  in  the  distal  parts  and  in  the  origin 
of  the  main  trunks,  which  may  arise  from  the  dorsal  aorta  or  from  the 
radices  as  far  forward  as  the  third  aortic  arch. 

The  main  trunk  of  the  fore  limb  may  have  different  names  in  differ- 
ent parts  of  its  course.  It  is  the  subclavian  artery  as  it  leaves  the 
dorsal  aorta,  the  axillary  as  it  enters  the  limb,  and  the  brachial  in  the 
upper  arm.  It  divides  near  the  elbow  into  radial  and  ulnar  arteries, 
which  run  near  the  corresponding  bones  into  the  podium. 

There  are  some  additional  elements  of  complexity  in  the  develop- 
ment of  the  arteries  of  the  hind  leg.  As  in  front  several  somatic  vessels 
are  concerned  and  there  is  the  same  formation  of  a  capillary  network. 
Two  of  the  arteries  attain  special  prominence.  In  front  is  the  epigas- 
tric artery,  which  descends  from  the  aorta  to  the  ventral  side  of  the 
body  and  runs  forward  to  supply  the  lower  portion  of  the  myotomes, 
becoming  connected  at  first  with  the  epigastric  veins,  although  later 
they  may  anastomose  with  the  hinder  ends  of  the  cutaneous  arteries 
(infra) .  When  the  hind  limb  grows  out,  the  epigastric  sends  a  branch, 
the  external  iliac  or  femoral  artery,  into  its  anterior  side.  As  the 
leg  increases  in  size  this  may  surpass  the  parent  epigastric  in  size,  the 
latter  now  appearing  as  a  side  branch. 

The  second  pair  of  somatic  arteries  are  the  sciatic  (ischiadic) 
arteries.  These  descend  into  the  posterior  side  of  the  leg,  the  name 
changing  at  the  angle  of  the  knee  to  popliteal  artery,  and  farther 
down  it  divides  into  peroneal  and  anterior  and  posterior  tibial 
arteries,  the  peroneal  supplying  the  calf  of  the  leg,  the  others  continuing 
into  the  foot. 

The  arrangement  of  vessels  thus  outlined  is  characteristic  of  the 
lower  tetrapoda  where  the  femoral  artery  is  small.  It  is  also  character- 
istic of  the  embryos  of  the  mammals,  but  in  the  latter,  before  birth,  the 
femoral  artery  grows  down,  joins  the  popliteal,  and  thus  becomes  the 
chief  supply  of  the  limb.  These  trunks  and  the  hypogastric  do  not 
always  remain  distinct,  but  may  fuse  in  different  ways  at  the  base. 
Epigastric  and  hypogastric  arteries  are  distinct  in  many  reptiles  and  in 
birds,  but  elsewhere  they  fuse  to  form  the  common  iliac  artery,  so 
called  since  the  proximal  portion  of  the  femoral  is  often  called  the 
external,  the  hypogastric  the  internal  iliac  artery.     The  sciatic,  too, 


CIRCUXATORY   ORGANS.  289 

may  remain  distinct  or  it  may  fuse  with  the  others  at  the  base,  and 
then  its  independent  portion  appears  as  a  branch  of  the  common 
iliac  artery. 

The  dorsal  aorta,  which  continues  into  the  tail,  is  called  the  caudal 
artery  behind  the  point  where  the  sciatics  (common  iliacs)  arise. 

A  cutaneus  artery,  arising  from  either  the  subclavian  or  the 
pulmonary  artery  of  either  side  (both  conditions  occur  in  the  amphibia), 
runs  backward  in  the  skin  of  the  trunk,  and  may  extend  back  and  unite 
with  the  epigastric  artery.  When,  as  in  the  amphibia,  these  arise 
from  the  pulmonary  they  contain  venous  blood  and  the  skin  acts  as 
a  subsidiary  respiratory  organ  (p.  258). 

Veins. 

The  position  and  development  of  the  chief  longitudinal  venous 
trunks  have  already  been  outlined.  Both  these  and  other  veins  yet 
to  be  mentioned  frequently  undergo  shiftings  of  position  and  other 
modifications  during  growth,  but  before  describing  these  changes  some 
other  vessels  must  be  described. 

With  the  development  of  the  limbs  corresponding  veins  arise  (fig. 
294),  a  subclavian  vein  for  each  fore  limb,  a  common  iliac  for  the 
hind  leg,  these  bringing  the  blood  from  the  appendage  to  the  trunk. 
In  the  young  each  subclavian  empties  into  the  postcardinal  of  the  same 
side,  but  in  the  adult  the  opening  may  shift  to  the  Cuvierian  duct. 
The  common  iliac  vein  likewise  empties  into  a  vein,  the  epigastric 
or  lateral  abdominal,  which  runs  forward  in  the  body  wall  to  connect 
with  either  the  postcardinal  or  the  duct  of  Cuvier  (fig.  294,  A).  This 
condition  obtains  throughout  life  in  some  elasmobranchs,  but  higher 
in  the  scale  the  iliac  vein,  while  retaining  its  connexion  w4th  the 
epigastric,  grows  toward  the  middle  line  and  joins  the  postcardinal  of 
the  same  side,  a  condition  which  is  permanent  in  amphibia  and  reptiles 
(fig.  294,  B,  C),  where  blood  coming  from  the  hind  limb  has  two 
routes  to  the  heart. 

The  epigastric  veins  of  the  two  sides  may  fuse  in  the  median  line 
in  front  (amphibia,  some  reptiles,  birds),  forming  an  anterior  ab- 
dominal vein  (fig.  294,  C)  which  reaches  the  heart  by  passing  through 
the  remains  of  the  ventral  mesentery  (ligamentnm  teres)  to  the  liver 
and  thence  forward.  A  similar  anterior  abdominal  vein  has  been 
described  in  Echidna  but  is  unknown  elsewhere  in  the  mammals. 
19 


290 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


In  the  fishes  the  vessels  of  the  appendages  are  but  slightly  developed, 
there  being  a  subclavian  vein  entering  the  Cuvierian  duct,  and  occa- 
sionally a  brachial  vein  which  may  empty  into  the  sinus  venosus.  In 
the  amphibia  a  cutaneus  magnus  vein  (fig.  302),  coming  from  the 
skin  of  the  trunk,  may  enter  the  subclavian,  while  in  all  tetrapoda  the 
subclavian,  after  leaving  the  limb,  receives  a  superficial  cephalic  and  an 
axillary  vein,  the  latter  changing  its  name  in  the  appendage  to  the 


Fig.  294. — Relations  and  modifications  of  the  post-  and  subcardinal,  abdominal  and 
postcaval  veins  in  different  stages  ot  the  amphibia.  In  A  the  veins  (il)  from  the  hind  Hmb 
return  directly  to  the  heart  by  the  lateral  abdominal  veins  {la),  while  the  blood  from  the 
tail  (c)  passes  by  way  of  the  subcardinals  (sc)  through  the  mesonephroi  to  the  postcardinals 
(pc).  In  B  the  lateral  abdominals  have  united  in  front  to  form  the  anterior  abdominal 
vein  (aa) ;  the  iHacs  have  sent  a  branch  to  the  postcardinals,  which  have  grown  back  to  join 
the  caudals,  while  the  subcardinals  have  lost  their  connexion  with  the  caudal  and  have 
acquired  one  with  the  postcava  (p),  a  backward  growth  from  the  sinus  venosus.  In  C  the 
postcardinals  have  been  interrupted,  the  posterior  half  of  each  now  forming  an  advehent 
vein  while  the  subcardinals,  as  in  B,  form  the  revehent  veins  (r). 

brachial  vein.  In  the  hind  limb  the  common  iliac  vein  is  formed  by 
the  union  of  the  femoral  and  sciatic  (ischiadic)  veins,  as  well  as  the 
hypogastric  (internal  iliac)  vein  already  referred  to. 

In  the  classes  above  fishes  (dipnoi,  amphibia  and  amniotes)  a  new 
vein,  the  postcava  (vena  cava  inferior)  appears.  This  arises  in 
part  from  scattered  spaces,  in  part  as  a  diverticulum  of  the  sinus 
venosus  and  the  hepatic  veins,  and  grows  backward,  dorsal  to  the  liver, 
until  it  meets  and  fuses  with  the  right  subcardinal  vein  (fig.  295),  a 


CIRCULATORY  ORGANS. 


291 


portion  of  which  now  forms  a  new  trunk,  carrying  blood  from  the 
posterior  part  of  the  body  to  the  heart  (figs.  294,  295). 

With  the  appearance  of  the  postcava  changes  are  introduced  in  the 
embryonic  renal  portal  circulation  (  p.  280)  which  may  be  summarized 
as  follows :  The  subcardinals  lose  their  connexion  with  the  caudal  vein 
and  become  connected  with  each  other  by  transverse  vessels  (interrenal 
veins)  while  parts  of  the  postcardinals  adjacent  to  the  nephridial 
organs  separate  from  the  parts  in  front,  while  they  grow  backward 


om 


Fig.  295. — Development  of  postcaval  system  in  birds  {A,  B,  sparrow;  C,  D,  chick), 
schematized  after  A.  M.  Miller.  In  A  the  postcardinals  have  extended  nearly  to  the 
pelvic  region  and  the  subcardinals  are  appearing  as  isolated  spaces.  In  B  the 
subcardinal  spaces  are  uniting  and  the  capillary  system  connecting  with  the  postcardinals 
is  developing,  while  the  postcava  is  arising.  In  C  the  postcava  has  united  with  the 
subcardinal  of  the  right  side,  ai,  ischiadic  artery;  ate,  external  iUac  artery;  au,  imibilical 
(hypogastric)  artery;  da,  dorsal  aorta;  m,  mesonephric  veins;  om,  omphalomesenteric 
artery;  p,  postcava  and  its  anlagen;  sc,  subcardinal  and  its  elements;  vei,  external  iliac 
vein;  vi,  ischiadic  vein. 


and  connect  with  the  caudal  vein  (fig.  295).  These  posterior  parts 
of  the  postcardinals  now  become  the  advehent  veins  of  a  second 
renal  portal  system,  bringing  blood  from  the  tail  and  hind  limbs  to  the 
excretory  organs  (mesonephroi) .  The  subcardinals  of  the  two  sides 
usually  fuse  in  the  middle  line,  a  process  initiated  by  the  appearance 
of  the  interrenal  veins,  and  now  act  as  a  revehent  vessel,  carrying 
blood    from   the   excretory  organs  to  the  postcava  and  the  anterior 


292 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


portion  of  the  postcardinals  which  have  joined  the  anterior  ends  of  the 
subcardinals  (fig.  294,  C).  The  changes  in  the  postcardinals  and  the 
renal  portal  system  of  mammals  will  be  described  below. 

In  Ceratodus  (dipnoi,  fig.  296,  A)  there  are  some  differences  from  the  above 
account.  Thus  the  anterior  portion  of  the  right  postcardinal  (not  shown  in  the 
figure)  loses  its  connexion  with  the  vessels  behind  and  acts  as  a  vertebral  vein, 
taking  the  blood  from  the  intercostal  veins  of  that  side  back  to  the  heart.     The 


Fig.  296. — A,  venous  system  of  Ceratodus,  dorsal  view,  after  Spencer;  B,  of  a  urodele, 
ventral  view,  ab,  abdominal  vein;  av,  venae  advehentes;  b,  brachial;  c,  caudal;  cd,  Cuvierian 
duct;  ej,  external  jugular;  h,  heart;  hp,  hepatic  portal;  ij,  inferior  jugular;  j,  jugular;  il, 
iliac;  /,  Hver;  Ic,  lateral  cutaneus;  m,  mesonephros;  p,  ppstcava;  pc,  postcardinal;  r,  venae 
revehentes;  s,  subclavian;  /,  testes. 

caudal  and  the  subcardinals  form  a  continuous  trunk,  the  revehent  vessels  forming 
side  branches.  The  posterior  portions  of  the  postcardinals  grow  back  into  the 
tail  as  paired  vessels,  forming  no  connexion  with  the  caudal  vein.  In  Protopterus 
the  vertebral  vein  is  lacking,  the  subcardinals  are  not  fused  behind  while  the 
advehent  veins  are  connected  with  the  caudal. 


The  development  of  lungs  brings  about  the  appearance  of  one  or 
more  pairs  of  pulmonary  veins  which  bring  the  (arterial)  blood  from 
these  organs  to  the  heart.     These  arise  as  an  outgrowth  from  the 


CIRCULATORY   ORGANS.  293 

left  atrial  portion  of  the  heart,  dividing  farther  back  to  reach  the  two 
lungs.  At  no  time  do  the  pulmonary  veins  connect  with  the  sinus 
venosus,  but  they,  always  empty  into  the  left  atrium  (fig.  285). 

The  Foetal  Circulation. 

Some  features  of  the  foetal  circulation  of  the  amniotes  have  already 
been  alluded  to,  but  the  whole  may  be  summarized  here.  In 
the  amniotes,  with  the  development  of  a  large  yolk  sac  and  of  the 
allantois,  the  vessels  on  the  ventral  side  of  the  body  become  corre- 
spondingly modified.  The  processes  involved  may  be  readily  under- 
stood from  a  comparison  of  figs.  282  and  284.  The  yolk  sac  is  to  be 
regarded  as  a  diverticulum  of  the  intestine  while  the  allantois  is  a 
similar  outgrowth  from  the  urinary  bladder,  itself  a  process  of  the  ali- 
mentary canal.  These  outgrowths  naturally  carry  with  them  the  blood- 
vessels distributed  to  the  parts  from  which  they  arise.  Hence  the 
omphalomesenteric  artery  and  the  vitelline  veins  (derivatives  of  the 
omphalomesenteric  veins)  extend  out  ever  the  yolk,  increasing  in 
number  as  well  as  in  extent  of  their  branches  as  the  yolk  sac  spreads 
over  the  yolk. 

In  the  same  way  the  hypogastric  arteries  are  carried  out  with  the 
allantois,  these  portions  being  called  the  allantoic  or  umbilical 
arteries,  the  blood  being  carried  back  to  the  trunk  by  a  single  allan- 
toic vein.  These  two  kinds  of  vessels — arteries  and  veins — are  con- 
nected in  the  distal  part  of  the  allantois  by  a  rich  network  of  capillary 
vessels.  It  is  by  these  that  the  allantois  is  able  (p.  264)  to  act  in  the 
sauropsida  as  an  organ  of  respiration.  In  the  mammals,  by  means  of 
osmosis  through  the  placenta,  it  is  not  only  respiratory,  exchanging 
gases  with  the  uterine  walls  (there  is  no  exchange  of  blood  with  the 
mother),  but  they  serve  as  recipients  of  nourishment  by  the  passage 
of  plasma  from  the  maternal  tissues. 

From  the  foregoing  statements  it  will  be  seen  that  in  the  sauropsida 
five  vessels — three  arteries  and  two  veins — pass  out  through  the  um- 
bilicus to  the  foetal  adnexa,  but  in  the  mammals,  where  the  yolk  is 
wanting  and  the  yolk  sac  reduced  and  transitory  in  character,  the 
omphalomesenteric  artery  and  the  vitelline  vein  disappear  early,  leav- 
ing but  three  vessels  in  the  umbilical  cord.  In  the  elasmobranchs, 
where  there  is  a  large  yolk  sac  but  no  allantois,  only  the  yolk  sac  cir- 
culation is  found. 


294 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


Circulation  in  the  Separate  Classes. 


CYCLOSTOMES  present  marked  differences  in  the  circulation  of  the  two 
groups,  the  petromyzons  being  nearly  normal,  the  myxinoids-  decidedly  aberrant. 
The  aortic  arches  vary  in  number  with  the  number  of  gill  pouches  (p.  239).  In  the 
myxinoids  the  common  carotid  is  connected  with  all  of  the  efferent  branchials  by  a 


Fig.  297. — Oblique  ventral  view  of  venous  system  of  Petromyzon,  drawn  from  a  corro- 
sion preparation  (Princeton,  669);  ac,  precardinal;  c,  caudal;  gs,  genital  sinus;  hv,  hepatic 
vein;  ij,  inferior  jugular;  pc,  postcardinal;  sv^  sinus  venosus;  va,  ventral  aorta. 

trunk  running  parallel  to  the  body  axis,  just  dorsal  to  the  gill  pouches.  The  inter- 
segmental arteries  of  the  dorsal  region  are  irregular,  sometimes  alternating,  some- 
times appearing  in  pairs  on  the  two  sides  of  the  median  line.  In  the  myxinoids 
(fig.  297)  the  subcardinals  are  united  behind,  the  postcardinals  in  front,  these 
latter  uniting'  with  the  single  inferior  jugular  of  the  left  side  to  form  the  unpaired 
Cuverian  duct,  the  presence  of  which  renders  the  sinus  venosus  asymmetrical  and 


Fig.  298. — ^Anterior  arterial  vessels  of  the  tile  fish  (Lopholatilus),  after  Silvester,  a, 
auricle;  ab,  to  air  bladder;  am,  to  angle  of  mouth;  c,  coeHac  axis;  d,  dorsal  arteries;  da, 
dorsal  aorta;  ec,  external  carotid;  g,  genital  artery;  gs,  gastrosplenic;  h,  hyoid  artery;  ha, 
hepatic;  I,  lingual;  Ig,  left  gastric;  m,  mesenteric;  mh,  middle  hypobranchial;  0,  ophthalmic; 
pa,  parietal;  po,  postOrbital;  ps,  pseudobranch;  rg,  right  genital;  so,  supraorbital;  v,  ven- 
tricle; va,  ventral  aorta. 


forces  the  hepatic  veins  to  empty  into  the  right  side.  The  hepatic  portal  receives 
a  vein  from  the  head,  and  then  passes  back  to  a  contractile  portal  heart,  just  before 
it  enters  the  liver. 

FISHES. — In  the  fishes,  the  dipnoi  excepted,  the  circulation  corresponds  rather 
closely  in  its  main  features  with  the  primitive  condition  described  above.     The 


CIRCULATORY  ORGANS. 


295 


heart  is  purely  venous  and  the  only  peculiarities  to  be  mentioned  are  the  following: 
In  the  elasmobranchs  and  ganoids  the  valves  of  the  conus  are  arranged  in  several 
(3-8)  rows,  but  in  the  teleosts  (Buiyrinus  excepted)  they  are  reduced  to  a  single  row, 
apparently  corresponding  to  the  first  of  the  lower  forms.  In  the  latter  group  the 
bulbus  is  especially  well  developed.  The  aortic  arches  correspond  in  number  to 
the  functional  gill  slits — six  or  seven  in  the  notidanid  sharks,  five  in  other  elasmo- 
branchs and  at  most  four  in  ganoids  and  teleosts.  Paired  inferior  jugulars  are 
usually  present,  but  they  are  lacking  in  Polypterus,  while  in  Lepidosteus  and  many 
teleosts  they  are  united  into  a  single  trunk  emptying  direcdy  into  the  sinus  venosus. 
Epigastric  veins  are  usually  present  and  paired  but  are  absent  from  many  bony 
fishes. 


Fig.  299. — ^ Anterior  venous  system  and  heart  oiLopholaiiltis,  after  Silvester,  a,  auricle 
ab,  veins  from  air  bladder;  b,  bulbus;  bv,  brachial  vein;  c,  cerebral  vein;  cd,  Cuvierian  duct; 
cv,  caudal  vein;  d,  dorsal  branches  of  parietal  veins;/,  facial  vein;  g,  gastric  veins;  hp, 
hepatic  portal;  hv,  hepatic  veins;  ij,  inferior  jugular;  in,  is,  veins  from  intestine  and  spleen; 
/,  Uver;  pc,  postcardinal;  pd,  postcloacal;  per,  peritoneal;  ph,  pharyngeal;  po,  postorbital; 
re,  anterior  revehentes;  s,  sinus  venosus;  si,  veins  from  stomach  and  intestine;  th,  thyreoid; 
tnt,  thymus;  v,  ventricle;  va,  ventral  aorta;  vf,  vein  from  ventral  fin;  w,  outline  of  Wolfl&an 
body. 

DIPNOI. — In  this  group  the  atrium,  in  correlation  with  the  development  of 
lungs,  becomes  partially  divided  as  described  above.  No  true  atrio-ventricular 
valves  occur,  their  place  being  taken  by  a  strong  ridge  which,  in  systole,  closes  the 
canal  and  at  the  same  time  partially  divides  the  ventricle  into  arterial  and  venous 
halves.  The  conus  has  eight  rows  of  valves  and  in  Ceraiodus  the  tnincus  shows  the 
beginning  of  a  division  (completed  in  Protopterus)  separating  the  arterial  from  the 
venous  arches.     For  veins,  see  fig.  296. 

AMPHIBIA. — In  the  amphibia  the  division  of  the  atrium  by  a  septum  atriorum 
into  right  (venous)  and  left  (arterial)  halves  is  carried  farther.  This  septum  is 
fenestrate  in  urodeles  and  gymnophiones,  entire  in  anura,  but  in  none  is  it  carried 
clear  to  the  atrio-ventricular  wall.  In  systole  the  edge  of  the  septum  is  forced  for- 
ward, completely  separating  the  two  atria.  No  corresponding  septum  is  developed 
in  the  ventricle,  but  numerous  muscular  bands  extending  through  its  cavity  tend 
to  prevent  the  mingling  of  arterial  and  venous  blood.  In  Proteus,  Cryptobranchus 
and  the  caecilians  the  bulbus  is  simple  but  in  the  other  urodeles  and  the  anura  a 
spiral  septum  (possibly  representing  fused  valves)  is  developed  in  it,  separating  it 


296 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


into  two  tubes.  This  is  continued  in  the  anterior  part  of  the  truncus  by  a  horizontal 
septum  (short  in  urodeles,  longer  in  anura)  separating  aortic  and  pulmonary  trunks, 
the  former  subdivided  in  a  similar  way  a  little  farther  forward  into  carotid  and  aortic 
portions. 

In  the  early  larvae  of  the  amphibia  each  fully  developed  aortic  arch  except  the 
last  extends  into  the  gills,  but  as  the  branchiae  begin  to  be  absorbed,  a  small  vessel 
connecting  the  afferent  and  efferent  arteries  at  the  base  of  each  gill  enlarges  and 


Fig.  300. — Heart  and  adjacent  parts  of  Protopterus,  after  Rose  a,  atrium;  aoe, 
oesophageal  artery;  I,  air  bladder  (lung);  c,  conus;  h,  hepatic  vein;  ji,  is,  superior  and 
inferior  jugular  veins;  oe,  oesophagus;  pa,  pulmonary  artery;  pc,  postcardinal  vein;  ph, 
pharyngeal  artery;  s,  sinus  venosus;  sc,  subclavian  vein;  1-4,  afferent  branchial  (aortic) 
arteries. 


becomes  the  path  of  the  main  blood  stream  and  a  part  of  the  arch  of  the  adult  (fig. 
304).  Of  these  four  arches — ^3,  4,  5,  and  6  of  the  primitive  scheme — the  fifth  is 
lost  in  the  adults  of  all  except  a  few  urodeles  and  caecilians.  The  fourth  connects 
with  the  dorsal  aorta  and  the  sixth  with  the  pulmonary  arteries.  These  last,  which 
often  have  a  ductus  Botallii,  are  noticeable  for  the  large  cutaneus  arteries — anterior 
and  posterior — which  arise  from  them  and  which  play  an  important  part  in  respira- 


CIRCULATORY   ORGANS. 


297 


tion.  Connected  with  the  carotid  arteries  are  the  carotid  glands  (fig.  304).  In 
the  larval  stage  each  consists  of  a  network  of  blood-vessels — a  rete  mirabile — 
between  the  afferent  branchial  and  the  carotid  artery,  but  in  the  adult  this  degener- 


Tnaxsup 
maxtif 
intjuy 


Fig.  301, 


Fig.  302. 


ates  into  a  small  muscular  organ  containing  sympathetic  cells  (p.  165),  at  the 
base  of  the  carotid. 

The  postcava  is  well  developed  and  the  epigastric  veins  unite  to  form  an  anterior 


298 


COMPARATIVE   MORPHOLOGY    OF   VERTEBRATES. 


maxinf 
\  extmax 


vahd 


Fig.  303. 
Figs.  301,  302,  303. — Circulatory  system  of  Desmognathus  fuscus  after  Miss  Seelye; 
fig.  301,  superficial  vessels;  fig.  302,  deeper  vessels;  fig.  303,  vessels  of  the  dorsal  body 
wall;  all  from  the  ventral  surface,  aames,  mesenteric  arteries;  acut,  cutaneus  artery;  aduo, 
duodenal  artery;  aepig,  epigastric  artery;  ag,  artery  to  anal  gland;  agas,  gastric  arteries; 
ahep,  hepatic  artery;  ail,  iliac  artery;  aintcom,  communis  intestinal  artery;  ao,  aorta;  aoc, 
ocular  artery;  aph,  pharyngeal  artery;  an,  anus;  apul,  pulmonary  artery;  asc,  subclavian 
artery;  asp,  splenic  artery;  hi,  urinary  bladder;  ca,  caudal  artery;  cutmag,  cutaneus  major 
vein;  cutp,  cutaneus  parva  vein;  cv,  caudal  vein;  ec,  external  carotid;  extmax,  external 
maxillary;  ic,  internal  carotid;  ilv,  iliac  vein;  intcom,  conmion  intestinal;  intjug,  internal 
jugular;  intv,  intestinal  vein;  ling,  Ungual;  liv,  Uver;  maxinf,  maxsup,  inferior  and  superior 
maxillaries;  mn,  mesonephros;  ce,  oesophagus;  pc,  postcava;  r,  rectum;  spl,  spleen;  st, 
stomach;  sv,  sinus  venosus;  vabd,  abdominal  vein;  vcut,  cutaneus  vein;  vhp,  hepatic  vein; 
vert,  vertebral  artery;  vmes,  mesenteric  vein;  vp,  portal  vein;  vra,  vena  renalis  advehentis: 
vsp,  splenic  vein;  vves,  vein  from  bladder. 


CIRCULATORY  ORGANS. 


299 


abdominal  vein  (fig.  294),  while  the  blood  from  the  hind  limbs  may  return  to  the 
heart  through  either  the  anterior  abdominal  or  the  renal  portal  system. 

In  the  lungless  salamanders  (p.  258)  the  heart  and  blood-vessels  show  corre- 
sponding modifications.  There  is  no  septum  atriorum  and  the  pulmonary  arteries 
and  veins  fail  to  develop.  The  cutaneus  arteries  and  the  smaller  vessels  supplying 
the  pharyngeal  region  are  greatly  enlarged,  respiration  taking  place  through  the 
skin  and  the  mucous  membrane  of  the  throat. 

The  action  of  the  anuran  heart  may  be  out- 
lined here.  The  two  atria  contract  at  the  same 
time,  forcing  arterial  and  venous  blood  into  the 
ventricle,  but  it  is  kept  from  mixing  by  the  mus- 
cular bands  already  alluded  to.  At  the  systole 
of  the  ventricle  the  venous  blood,  which  is  near- 
est the  truncus,  is  first  forced  forward.  This 
takes  the  most  direct  course  through  the  wide 
and  shorter  pulmonary  arteries,  which  are  prac- 


FiG.  304.  Fig.  305. 

Fig.  304. — Diagram  of  the  aortic  arches  in  amphibia.  Arterial  blood  cross  lined, 
venous  black.  The  gill  circulation  omitted,  its  course  indicated  by  arrows;  the  permanent 
circulation  after  the  absorption  of  gills  shown,  eg,  carotid  gland;  da,  dorsal  aorta;  d, 
ductus  BotalU;  pa,  pulmonary  artery;  va,  ventral  aorta;  3-6,  aortic  arches. 

Fig.  305. — Heart  of  snapping  turtle,  Chelydra  serpentina  (Princeton,  479).  aa,  aortic 
arch;  c,  cceliac  artery;  da,  dorsal  aorta;  dh,  Botall's  duct;  ec,  ic,  external  and  internal 
carotids;  la,  left  auricle;  p,  pulmonary  artery;  ra,  right  auricle;  sc,  subclavian  artery;  v, 
ventricle;  m,  mesenteric  artery. 

tically  empty  at  the  time.  The  next  portion  of  the  blood,  containing  both  arterial 
and  venous,  follows  the  next  easiest  course  through  the  aortic  arches,  while  the 
last  to  leave  the  ventricle,  consisting  of  pure  arterial  blood,  can  only  go  into  the 
carotids,  where  the  resistance  is  greater  on  account  of  the  small  size  of  the  vessels 
and  the  obstacles  presented  by  the  carotid  glands. 

REPTILES. — In  the  reptiles  the  division  of  the  heart  (fig.  287)  is  carried  still 
farther  and  the  sinus  venosus  tends  to  be  merged  in  the  right  atrium.  The  atrial 
septum  is  complete  and  is  continued  forward  as  a  ventricular  septum,  partially 


300  COMPARATIVE   MORPHOLOGY    OF   VERTEBRATES. 

(Sphenodon,  turtles,  squamata)  or  completely  (crocodiles)  separating  the  two 
ventricles.  The  peculiar  relations  of  the  aortic  arches  have  been  mentioned  (p. 
283).  Correlated  with  the  dififerences  between  the  aortic  (fourth)  arches  of  the 
two  sides  in  the  majority  of  reptiles  are  certain  features  in  the  origin  of  the  arteries. 
Thus  both  of  the  subclavian  arteries  (lacking  in  snakes)  arise  from  the  right  radix, 
while  the  left  gives  rise  to  the  coeliac  artery.  In  many  reptiles  the  anterior  parts  of 
the  postcardinals  are  replaced  by  vertebral  veins.  The  renal  portal  system  is 
developed  in  the  embryo  and  persists  (much  as  in  the  amphibia)  to  a  greater  or  less 
extent  in  the  adult.     Usually  paired 'anterior  abdominal  veins  are  present. 

BIRDS. — The  peculiarities  of  the  heart  and  aortic  arches  were  mentioned  on 
page  283.  Birds  have  the  same  reduction  of  the  postcardinals  as  is  found  in  reptiles. 
The  renal  portal  system  is  formed  in  the  embryo,  but  the  only  blood  received  by 
the  adult  kidney  comes  through  renal  arteries  like  those  of  mammals.  The  iliac 
veins  extend  to  the  postcava  and  lose  all  connexion  with  the  anterior  abdominal 
veins.     The  paired  epigastric  veins  persist  only  in  front. 

MAMMALS. — In  the  mammals  the  four  chambers  of  the  heart  are  completely 
separated  and  the  sinus  venosus  has  been  completely  merged  in  the  right  atriumt 
The  persistent  left  fourth  aortic  arch  forms  the  sole  connexion  between  the  hear. 

Fig.  306. — Modifications  of  the  origin  of  the  carotid  and  subclavian  arteries  in 

mammals. 

and  the  dorsal  aorta  and  from  it  arise  the  carotid  and  subclavian  arteries,  the 
arrangement  of  these  representing  almost  every  possible  condition  (fig.  306).  In 
the  lower  groups  {e.g.,  rodents)  both  Cuvierian  ducts  persist,  but  in  the  higher  orders 
a  cross  connexion  (the  innominate  vein)  arises  between  the  trunks  formed  from 
the  jugulars  and  subclavian  veins  of  the  two  sides  (fig.  308)  so  that  the  blood  from 
the  left  side  of  the  head,  neck  and  fore  limb  joins  that  of  the  left  side  in  a  common 
trunk,  the  precava  (anterior  vena  cava)  which  enters  the  right  atrium.  With 
this  development  the  left  Cuvierian  duct,  as  such,  disappears. 

The  renal  portal  system  has  but  a  transitory  existence  in  the  embryo  (best 
developed  in  the  monotremes)  and  early  disappears  with  the  degeneration  of  the 
Wolffian  bodies  (mesonephroi).  As  these  organs  disappear  a  part  of  the  capillary 
system  of  the  Wolffian  bodies  enlarges  and  forms  a  main  trunk  connecting  the 
postcava  with  the  posterior  parts  of  the  postcardinal  veins  (fig.  307,  C)  which  bring 
the  blood  from  the  tail,  the  iliacs  and  the  permanent  kidneys.  With  farther  develop- 
ment (Z>,  E)  the  left  postcardinal  is  largely  lost  (except  the  part  connecting  with  the 
suprarenal  and  gonad  of  that  side)  and  all  the  blood  from  the  posterior  part  of  the 
body  is  returned  by  the  right  postcardinal  and  the  postcava,  which  appear  (fig. 
308,  A)  as  if  they  arose  from  a  union  of  the  iliac  veins.  Correlated  with  these 
changes  in  the  venous  system  and  the  impossibility  of  venous  blood  entering  the 
excretory  organs,  there  is  developed  a  renal  artery  from  the  aorta  for  each  of  the 
permanent  kidneys. 


CIRCULATORY   ORGANS. 


301 


Fig.  307. — Development  of  posterior  veins  of  rabbit,  after  Hochstetter.  C  and  D  repre- 
sent only  the  hinder  part  of  the  whole  shown  in  A  to  C  In  B  the  veins  for  the  postcaval- 
subcardinal  system  have  tapped  the  postcardinal  veins,  which  in  C  have  lost  their  connec- 
tion with  the  anterior  part  and  empty  now  through  the  postcava  exclusively.  In  E  the 
left  posterior  postcardinal  is  entirely  lost,  i,  ischiadic  vein;  ie,  external  iliac;  i,  jugular; 
nU,  metanephros  (kidney);  p,  postcava;  pc,  postcardinal;  s,  subclavian;  sc,  subcardinal: 
sr,  suprarenal;  m,  ureter. 


Fig.  308. — Development  of  the  anterior  veins  of  a  mammal.  A,  earlier  stage,  to  be 
compared  with  fig.  307  C;  B,  definitive  condition  of  adult,  a,  azygos;  c,  coronary;  e,  i, 
external  and  internal  jugular;  ha,  hemiazygos;  il,  iliac;  in,  innominate;  p,  postcava;  pc 
postcardinal;  pre,  precava  (superior  vena  cava);  si,  superior  intercostal  veins. 


302        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

The  anterior  parts  of  both  postcardinals  have  separated  from  the  posterior  por 
tion  and  receive  only  blood  coming  from  the  intercostal  veins  (fig.  308).  A 
cross  vessel  now  connects  the  posterior  parts  of  the  postcardinals  of  the  two  sides, 
after  which  the  left  vessel  separates  into  two  portions.  The  anterior  of  these 
(fig.  308,  B)  connects  with  the  heart  by  way  of  the  jugular  and  innominate  vein  and 
forms  the  superior  intercostal  vein  of  human  anatomy.  The  rest  of  the  left 
postcardinal  is  now  known  as  the  hemiazygos  vein  and  it  returns  blood  from  the 
trunk  by  way  of  a  cross  connexion  and  the  anterior  part  of  the  right  postcardinal 
(now  called  the  azygos  vein),  to  the  precava  and  so  to  the  heart. 

THE  LYMPHATIC  SYSTEM. 

The  lymphatic  system  consists  of  (i)  a  series  of  lymph  vessels  which 
penetrate  all  parts  of  the  body;  (2)  of  pulsating  portions  of  these  vessels, 
the  lymph  hearts;  and  (3)  peculiar  aggregates  of  connective  tissue, 
leucocytes  and  lymph  vessels  which  are  grouped  under  the  general 
head  of  Ijrmph  glands. 

There  are  different  views  as  to  the  morphology  of  the  blood  and  lymph  systems. 
According  to  one  (Marcus)  the  lymph  vessels  were  primitively  connected  with  the 
coelom  and  have  only  secondarily  come  into  relations  with  the  blood-vascular 
system.  Others  think  that  both  blood  and  lymph  vessels  have  arisen  from 
extraccelomic  spaces,  from  which,  by  modification  and  specialization,  the  two 
systems  have  been  differentiated.  The  fact  that  in  many  invertebrates  there  is 
but  a  single  system,  best  compared  with  the  lymph  system  of  the  vertebrates,  and 
that,  even  in  the  Crustacea,  lymphatic  and  blood  systems  are  but  partially  differ- 
entiated, is  of  interest  in  this  connexion. 

The  lymph  vessels  are,  in  part,  capillary  in  character  with  walls  of 
endothelium  alone.  The  larger  ducts  and  the  still  larger  sinuses  are 
strengthened  by  smooth  muscle  fibres  and  by  elastic  and  fibrous  tissue. 
The  capillaries  have  numerous  anastomoses,  but  the  vessels  are  said 
to  terminate  blindly,  while,  at  least  in  the  higher  vertebrates,  some  may 
connect  with  the  coelom  by  minute  openings  (stomata)  in  the  peritoneal 
lining.  The  larger  vessels  have  valves  at  intervals  to  prevent  back- 
flow  of  the  lymph,  these  often  giving  the  vessels  a  lobulated  appearance. 
Proximally  the  vessels  open  at  two  or  more  points  into  the  veins.  The 
fluid  portion  of  the  lymph  is  derived  in  part  by  osmose  from  the  walls  of 
the  blood  capillaries,  in  part  from  the  alimentary  canal. 

The  development  of  the  lymph  vessels  has  been  traced  mainly 
in  birds  and  mammals  (chiefly  in  the  latter),  with  fewer  observations  on 
amphibia  and  other  classes.  Many  points  remain  to  be  worked  out, 
there  being  considerable  differences  in  the  various  accounts.  Appar- 
ently the  process  in  its  main  features  is  as  follows: 


CIRCULATORY   SYSTEM. 


303 


Near  the  junction  of  pre-  and  postcardinals  on  either  side  numerous 
small  diverticula  are  given  off  from  the  lateral  side  of  these  veins  (fig. 
309,  A).  These  diverticula  unite  with  each  other,  forming  small 
tubes  parallel  to  the  parent  vessels  and  united  to  them  for  a  time  at 
numerous  points  where  the  budding  took  place.  Later  these  connex- 
ions are  lost  and  the  tubes  are  separated  from  the  veins  (fig.  309,  B) 
forming  an  anterior  cephalic  duct,  extending  forward,  parallel  to  the 
jugular  vein;  an  ulnar  lymphatic  duct  destined  to  grow  into  the  fore 
limb;  and,  a  little  later,  a  thoracic  duct  grows  back,  parallel  to  the 


Fig.  309. — Early  development  of  the  lymph  vessels  in  the  cat,  after  McCliire  and 
Huntington.  A,  in  6.5  mm.  embryo;  B,  in  10.5  mm.  embryo;  C,  definitive  stage;  D, 
diagram  of  developing  diverticula  of  chick  which  are  to  form  lymph  heart,  based  on  Sala. 
ac,  anterior  cardinal  vein;  c  ^-^,  coccygeal  veins;  cd,  Cuverian  duct;  cv,  cephalic  vein;  dls, 
dorsal  veno-lymphatic  sinus;  ej,  ij,  external  and  internal  jugulars;  pre,  precava;  th,  thoracic 
duct;  ul,  primitive  ulnar  lymphatic;  uva,  anlage  of  ulnar  vein;  vis,  ventral  veno-lymphatic 
sinus;  1-7,  segmental  vessels;  lymphatic-forming  tissue  stippled. 

postcardinal  vein.  All  of  these  vessels  are  united  near  their  point  of 
origin  by  a  large  sinus,  the  jugular  lymph  sac  (fig.  309,  C).  Later 
the  lymph  sac  reestablishes  communication  at  one  or  two  points  in  the 
subclavian-jugular  region  with  the  vein. 

The  conditions  at  the  posterior  part  of  the  body  are  less  certainly 
known  (fig.  309,  D).  In  this  region  a  cistern  of  chyle  (a  mesenterial 
lymph  sac)  and  a  posterior  lymph  sac  develop  in  close  connexion 
with  the  postcava  in  the  region  of  the  nephridial  organs,  and  it  is  pos- 
sible that  a  portion  of  the  thoracic  duct  grows  forward  from  the  cis- 
tern of  chyle,  while  other  vessels  grow  into  other  regions.  Later  the 
primitive  trunks  thus  outlined  give  off  branches  which  gradually  ex- 


304 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


tend  into  all  parts  of  the  body,  but  of  their  development  little  is  known. 
Anastomoses  occur  between  the  vessels  of  the  two  sides  of  the  body 
and  not  infrequently  the  thoracic  duct  of  one  side  shows  more  or  less 
degeneration,  resulting  in  a  lack  of  symmetry  in  the  adult. 

Not  enough  is  known  of  the  distribution  of  the  lymphatic  trunks  to 
render  broad  generalizations  possible,  but  it  may  be  said  that  the  sys- 
tem is  most  extensively  developed  in  the  subcutaneous  tissue,  in  the 
corresponding  envelopes  (meninges)  of  the  central  nervous  system, 
in  the  intermuscular  connective  tissue,  in  the  walls 
of  the  alimentary  canal,  and,  as  a  network,  in  close 
connexion  with  the  blood-vessels  of  the  body. 

The  l3miph  hearts  are  enlarged  and  contrac- 
tile portions  of  the  lymph  vessels,  provided  with 
valves  to  prevent  backflow  of  the  fluid  (fig.  310). 
Usually  these  contract  by  means  of  the  intrinsic 
muscles  of  the  walls,  but  in  some  urodeles  {Am- 
hly stoma)  there  is  an  unpaired  lymph  heart  beneath 
the  truncus  arteriosus  which  enlarges  and  con- 
tracts with  the  systole  and  diastole  of  the  blood 
heart. 

As  was  intimated  above  there  is  a  constant 
osmosis  of  fluid  from  the  blood  capillaries  into 
the  surrounding  tissues.  This  finally  passes  into 
the  distal  capillaries  of  the  lymph  system,  while 
in  the  walls  of  the  alimentary  canal  there  are,  in 
addition,  the  results  of  the  digestive  processes 
added  to  the  fluid  in  the  lymph  vessels.  As  this  latter  portion  has  a 
milky  appearance,  due  to  the  contained  fat,  it  is  called  chyle  and 
the  lymphatics  which  contain  it  are  called  lacteals  and  chyle 
ducts.  All  of  these  additions  to  the  contents  of  the  lymph  vessels 
make  a  current  in  the  larger  lymph  trunks,  and  finally  the  whole 
of  the  lymph  is  returned  to  the  veins  by  the  several  connexions  already 
mentioned.  In  addition  to  the  propelling  force  of  the  lymph  hearts  and 
the  pressure  due  to  absorption  and  osmosis,  the  lymph  is  also  carried 
along  by  the  motions  of  the  parts  in  which  the  vessels  ramify,  their 
pressure  being  supplemented  by  the  action  of  the  valves. 

In  those  fishes  which  have  been  accurately  studied  the  lymph  system  is  well 
developed  and  opens  into  the  veins  in  the  cardiac  and  caudal  regions.  The  vessels 
are  especially  developed  in  the  tail,  where  (myxinoids,  teleosts)  lymph  hearts  occur. 


Fig.  310. — Scheme 
of  caudal  lymph  heart 
of  teleost,  after  Favaro. 
a,  atrium;  /,  lymph  ves- 
sels; Is,  lymph  sinus;  v^ 
ventricle;  vs,  venous 
sinus  of  caudal  vein. 


CIRCULATORY   ORGANS. 


305 


There  is  also  a  large  lymph  sinus  in  the  scapular  region  into  which  the  trunks  from 
head  and  body  empty.  Frequently  there  is  also  a  large  caudal  sinus  (physostomes) 
connected  with  a  lymph  heart  (fig.  310)  which  forces  the  lymph  into  the  caudal 
vein. 

The  urodeles  have  the  thoracic  ducts  united  behind  but  separate  in  front,  a 
cephalic  trunk  emptying  into  each,  and  each  duct  opening  into  the  corresponding 
subclavian  vein,  while  a  series  of  from  fourteen  to  twenty  lymph  hfearts  occur  in 
connexion  with  the  trunk  accompanying  the  lateral  line.  The  anura  are  noticeable 
for  the  complete  disappearance  of  the  thoracic  ducts,  their  place  being  taken  by  a 


Fig.  311. — Deeper  anterior  lymphatics  (stippled)  of  Scorpenichthys,  after  Allen,  a, 
auricle;  ahs,  abdominal  sinus;  h,  brachial  sinus;  hr,  brain;  cs,  cephalic  sinus;  d,  dorsal  trunk; 
fm,  facialis-mandibularis  vein;  hs,  hyoid  sinus;  ij,  inferior  jugular  vein;  ips,  inner  pectoral 
fin  sinus;  ;',  jugular  vein;  /,  lateral  trunk;  on,  orbito-nasal  vein;  p,  pericardial  sinus;  pf^ 
profundus  facialis  lateral  trunk;  pv,  profimdus  ventral  trunk;  sf,  superficial  lateral  trunk; 
55/,  superior  spinal  longitudinal  trunk;  v,  ventricle;  va,  ventral  aorta;  vfs,  ventral  fin  sinus; 
vp,  ventral  pericardial  sinus;  vt,  ventral  abdominal  trunk. 


pair  of  trunks  between  the  dorsal  myotomes  and  those  of  the  lateral  body  wall. 
They  have  also  enormous  subcutaneous  lymph  spaces,  separated  from  each  other 
by  narrow  partitions.  It  is  the  presence  of  these  large  spaces  that  makes  the  skin- 
ning of  a  frog  such  an  easy  matter.  Two  pairs  of  lymph  hearts  are  present,  one 
pair  in  the  neighborhood  of  the  extremity  of  the  urostyle,  the  other  between  the 
transverse  processes  of  the  third  and  fourth  vertebrae.  In  the  caecilians  there  is  a 
pair  of  lymph  hearts  for  each  segment  of  the  trunk. 

Reptiles  have  two  cephalic  lymph  trunks  and  one  (lizards)  or  two  thoracic 


306  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

ducts,  the  one  of  the  lizards  being  divided  in  front  so  as  to  empty  into  either  sub- 
clavian vein.  There  is  a  single  lymph  heart  at  the  junction  of  trunk  and  tail.  In 
the  birds  both  thoracic  ducts  occur  and  there  is  a  pair  of  lymph  hearts  present  in 
the  young  in  the  position  occupied  by  the  single  heart  of  the  reptiles. 

In  the  mammals  the  primitively  paired  thoracic  ducts  are  sometime  retained 
throughout  life,  but  usually  only  one  persists.  This  begins  at  the  cistern  of  chyle 
in  the  lumbar  region  and  empties  into  the  left  brachiocephalic  vein  near  the  entrance 
of  the  single  cephalic  duct.  The  thoracic  duct  receives  the  lymph  vessels  from  the 
limbs  and  those  (lacteals)  from  the  alimentary  canal.     In  those  cases  where  there 


Fig.  312. — Early  lymph  system  (black)  of  10  mm.  rabbit  embryo,  after  F.  T.  Lewis. 
Sit,  anterior  tibial;  c,  caudal;/j,  primitive  fibular;  ej,  ij,  external  and  internal  jugular;  em, 
external  mammary;  pc,  postcardinal;  ul,  primitive  ulnar  veins. 

is  but  a  single  thoracic  duct  in  front,  its  representative  on  the  right  side  is  a  much 
smaller  vessel  connected  with  the  right  side  of  the  venous  system.  No  lymph 
hearts  are  known  in  the  mammals.  The  jugular  lymph  sacs  of  the  embryo  have 
been  regarded  as  such,  but  the  absence  of  valves  and  muscles  in  the  walls  renders 
such  an  interpretation  doubtful. 

Lymph  Glands. — In  connexion  with  the  lymph  vessels  are  numer- 
ous structures  included  under  the  heads  of  lymph  glands,  lymph 
nodules  and  hlood-lymph  glands.  These  are  most  abundant  in  the 
walls  of  the  coelom  (mesenteries)  and  of  the  digestive  tract,  although 
they  may  be  found  at  remote  points.     They  consist  of  aggregates  of 


UROGENITAL  SYSTEM.  307 

adenoid  tissue  (reticular  connective  tissue  crowded  with  leucocytes). 
Well-known  among  these  structures  are  the  so-called  fat  bodies  (cor- 
pora adiposa)  connected  with  the  gonads  of  the  amphibia,  and  the 
'hibernating  glands'  of  some  rodents  and  insectivores,  which  con- 
sist of  richly  vascularized  masses  of  fat.  In  the  lymph  nodules  this 
adenoid  tissue  is  enmeshed  in  a  rete  mirabile  of  lymph  vessels.  In 
the  blood-ljrmph  glands  there  is  a  somewhat  similar  relation  to  blood- 
vessels as  well,  for  the  details  of  which  reference  must  be  made  to 
histological  text-books.  These  lymph  structures,  which  occur  at 
various  points  of  the  body,  are  apparently  places  for  the  formation  of 
leucocytes  (lymphocytes) . 

The  spleen,  attached  to  the  mesentery  near  the  stomach  and  pan- 
creas, is  intermediate  in  some  respects  between  lymph  and  blood- 
lymph  glands  and  is  the  largest  lymph  structure  in  vertebrates.  It 
is  developed  in  the  walls  of  the  alimentary  canal  and  is  said  to  have 
an  entodermal  origin.  Later  it  separates  from  the  stomach  and 
assumes  its  definitive  position.  It  serves,  apparently,  as  a  place  for 
the  disintegration  of  the  red  blood  corpuscles  in  addition  to  functioning 
as  a  leucocyte-forming  organ. 

The  tonsils  (p.  247)  belong  to  the  category  of  adenoid  glands. 
There  are  two  kinds  of  these,  the  pharyngeal  and  the  palatine  tonsils, 
the  latter  occurring  between  the  inner  ends  of  the  Eustachian  tubes  of 
amniotes,  the  palatine  (best  developed  in  mammals)  are  paired  struc- 
tures on  either  side  of  the  pillars  of  the  fauces.  Other  tonsil-like 
structures  occur  at  different  points  of  the  floor  and  roof  of  the  mouth 
of  the  tetrapoda. 

THE  UROGENITAL  SYSTEM. 

In  several  phyla  of  the  animal  kingdom  there  is  an  intimate  relation 
between  the  reproductive  and  excretory  organs,  the  ducts  of  the  latter 
sendng  either  to  carry  the  products  of  the  gonads  directly  to  the  ex- 
terior or  acting  as  brood  organs  where  a  portion  of  the  development 
of  the  egg  takes  place.  This  close  association  of  the  two  systems  is 
especially  marked  in  most  vertebrates  and  hence  this  section  is  headed 
Urogenital  System,  because  of  the  diflBiculty  of  treating  the  two  com- 
ponents separately. 

The  urinary  or  excretory  organs  have  for  their  purpose  the  elimina- 
tion of  the  nitrogenous  waste  (and  occasionally  other  products)  from 


308         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

the  system.  They  are  paired  organs  which  consist  of  glandular  por- 
tions, the  nephridia  (kidneys),  and  their  ducts.  The  reproductive 
organs  include  the  gonads  or  sexual  'glands,'  which  (ovaries)  pro- 
duce the  eggs  or  (testes)  the  spermatozoa,  and  the  passages  by  which 
these  products  are  carried  to  the  external  world.  To  these  are  fre- 
quently to  be  added  accessory  reproductive  structures  by  which,  in 
certain  cases,  the  sperm  is  transferred  to  the  female. 


Fig.  313. — Urogenital  organs  of  Emys  europea,  after  Bojanus.  b,  urinary  bladder; 
g,  opening  of  vas  deferens  into  the  urogenital  sinus;  k,  kidney;  t,  testis;  u,  ureter;  vd,  vas 
deferens. 


THE  EXCRETORY  ORGANS. 

The  nephridia  consist  of  a  series  of  excretory  tubules,  specialized 
in  different  ways,  and  of  the  ducts  into  which  the  tubules  empty.  As 
the  function  of  the  nephridia  is  the  elimination  of  the  nitrogenous 
waste  (uric  acid,  urea,  etc.)  which  accumulates  in  the  blood,  they  have 
an  abundant  blood  supply,  entirely  derived,  in  the  younger  stages  of 
all  vertebrates  and  in  the  adults  of  the  higher  groups  from  the  dorsal 
aorta,  while  in  the  later  developmental  stages  and  in  the  adults  of  most 
anamniotes  the  aortic  blood  is  supplemented  by  blood  coming  from 
the  tail  and  hind  limbs  by  way  of  the  caudal  and  iliac  veins  (fig.  303). 

In  its  extreme  development  one  of  the  excretory  tubules  may  con- 
sist of  the  following  parts  (fig.  314):  At  the  proximal  end  the  tubule 
opens  into  the  ccelom  (metacoele)  by  a  ciliated  funnel,  the  nephro- 


UROGENITAL   SYSTEM. 


309 


stome;  the  cilia,  which  may  continue  for  some  distance  along  the 
inside  of  the  tubule,  serving  to  create  a  current  which  carries  the 
coelomic  fluid  into  the  tubule  and  thence  outward.  Farther  along 
the  tubule  expands  into  a  Malpighian  or  renal  corpuscle  (fig.  315). 
This  consists  of  a  vesicle  (Bowman's  capsule),  one  side  of  which 


Fig.  314. — Diagram  of  conventionalized  excretory  tubule.  a,  ascending  limb  of 
Henle's  loop;  h,  Bowman's  capsule  of  Malpighian  body;  c' — c^,  first  and  second  con- 
voluted tubules;  ct,  collecting  tubule;  d,  descending  limb  of  Henle's  loop;  g,  glomerulus 
of  Malpighian  body;  wnth  artery  and  vein;  h,  Henle's  loop;  n,  nephrostome  opening  into 
coelom;  x,  entrance  of  other  tubules  into  collecting  duct. 

projects  into  the  other,  nearly  filling  the  cavity.  This  intumed  portion 
is  the  glomerulus.  It  consists  of  a  network  of  capillary  blood-vessels, 
supplied  by  an  artery  and  drained  by  a  vein.  Beyond  the  connexion 
of  the  Malpighian  body  the  tubule  becomes  contorted  or  convoluted 
and  its  cells  are  strongly  glandular  in  character.  This  first  convoluted 
tubule  is  succeeded  by  a  nearly  straight  tract,  folded  once  on  itself  into 


Fig.  315. 


-Diagram  of  renal  (Malpighian)  corpuscle,     a,  artery;  h.  Bowman's  capsule; 
gl,  glomerulus;  «,  nephrostome;  /,  nephridial  tubule;  v,  vein. 


the  descending  and  ascending  limbs  of  Henle's  loop.  Next  follows 
the  second  convoluted  tubule,  which  passes  by  means  of  a  short 
connecting  tubule  into  a  non-glandular  collecting  tubule  into  which 
several  other  systems  of  excretory  tubules  enter,  and  which  leads 
more  or  less  directly  into  the  urinary  duct  which  conveys  the  waste 
from  the  excretory  organ  to  the  exterior. 


3IO        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

One  or  another  of  these  typical  parts  may  be  lacking  in  certain 
groups.  Thus  in  the  amniotes  the  nephrostomes  are  never  formed, 
though  they  occur  in  most  ichthyopsida.  In  the  pronephros  the 
Malpighian  corpuscle  is  rudimentary  or  lacking  at  all  stages  while 
there  is  no  differentiation  of  convoluted  tubules  and  Henle's  loop. 

The  function  of  the  various  parts  of  the  nephridial  tubule  is  in 
outline  as  follows:  Theoretically  it  would  appear  that  in  the  primitive 
condition  the  nitrogenous  waste,  which  is  elaborated  in  the  liver, 
collected  in  the  coelom  and,  together  with  the  coelomic  fluid,  was 
passed  outward  through  the  nephrostomes  and  the  tubules,  which 
acted  merely  as  ducts.  Higher  in  the  scale  the  parts  become  more 
differentiated  and  specialized.  The  renal  corpuscles  form  a  filtering 
apparatus  by  which  water  is  passed  from  the  blood-vessels  of  the  glom- 
erulus into  the  tubules  near  their  beginning,  and  this  serves  to  carry 
out  the  urea,  uric  acid,  etc.,  secreted  by  the  glandular  portions  of  the 
walls  of  the  tubules  (convoluted  tubules,  ascending  limb  of  Henle's 
loop). 

In  development  there  may  be  three  successive  series  of  nephridial 
structures,  the  higher  number  occurring  only  in  the  amniotes.  These 
are  known  as  the  pronephros  (head  kidney),  mesonephros  (Wolffian 
body),  and  the  metanephros  (permanent  kidney  of  the  amniotes). 
All  three  are  closely  related  in  development  and  structure  but  are 
distinguished  by  differences  in  origin  and  in  the  final  details.  Three 
views  are  held  as  to  their  relations  one  to  another.  According  to  one 
they  are  parts  of  an  originally  continuous  excretory  organ  (holone- 
phros)  which  extended  the  length  of  the  body  cavity.  This  has  be- 
come broken  up  into  the  separate  parts  which  differ  merely  in  time 
of  development  and  function,  with  minor  modifications  in  details.  A 
second  view  is  that  they  are  three  separate  organs,  while  a  third  regards 
them  as  superimposed  structures  which  occasionally  overlap  (birds, 
gymnophiona)  and  thus  are  not,  strictly  speaking,  homologous  but 
rather  homodynamous.  The  first  view  has  the  most  in  its  support, 
but  for  convenience  the  three  structures  are  kept  distinct  here.  All 
arise  from  the  mesomeric  somites  or  from  the  Wolffian  ridge  which 
appears  on  either  side  of  the  median  line  where  the  mesomeres  separate 
from  the  rest  of  the  wall  of  the  body  cavity,  the  mesomeric  cells  furnish- 
ing the  nephrogenous  tissue  from  which  the  definitive  organs  develop. 

Pronephros. — ^The  pronephros  is  the  first  to  appear  in  develop- 
ment.    As  will  be  recalled  (p.  14)  the  mesomere,  like  the  epimere, 


UROGENITAL   SYSTEM. 


311 


becomes  segmented,  and  later,  when  the  epimere  separates  to  form 
the  myotome,  the  dorsal  end  of  each  mesomere  becomes  closed,  the 
whole  then  forming  a  sac,  opening  below  into  the  ventral,  undivided 
coelom  (metacoele).  A  varying  number  of  these  nephrotomes  (as 
they  are  called)  lying  a  little  behind  the  head  are  concerned  in  the 


Fig.  316. — Scheme  of  origin  of  pronephric  tubules  after  Felix.  .4,  earlier,  B,  later 
stage,  c,  coelom;  d,  pronephric  tubule  and  duct;  e,  epimere;  h,  hypomere;  w,  mesomere 
(lined);  n,  nephrostome;  my,  myotome;  so,  sp,  somato-  and  splanchnopleure. 

formation  of  the  pronephros  (two  in  most  urodeles  and  amniotes; 
three  in  lampreys,  anura,  some  sharks  and  some  anmiotes;  four  or 
five  in  some  sharks  £ind  Lepidosteus;  seven  or  eight  in  skates;  eight  to 
eleven  in  Amia;  and  a  dozen  in  some  caecilians;  while  it  is  claimed  that 
the  whole  series  of  nephridial  tubules  of  Bdellostoma  is  pronephric). 
The  somatic  wall  of  these  nephrotomes  (fig.  316)  grow  out  toward 


Fig.  317. — Reconstruction  from  longitudinal  sections  of  pronephros  of  Hypogecphis 
(caecilian),  after  Brauer.  Pronephric  duct  {pd)  and  primary  pronephric  tubules  light; 
the  rest  of  the  somites  (nephrotomes)  black;  glomeruli  between  tubules  2-8.  The  three 
trunk  somites  in  front  of  i  develop  no  tubules. 

the  ectoderm,  thus  forming  slender  pronephric  tubules  (or  solid  cords 
which  later  become  canalized),  the  proximal  end  of  each  communica- 
ting freely  with  the  metacoele  by  way  of  the  cavity  of  the  nephrotome, 
the  opening  of  the  latter  into  the  metacoele  being  the  nephrostome. 
As  will  be  understood,  these  tubules,  like  the  nephrotomes,  are  meta- 
meric  in  character,  equalling  the  somites  in  number.    The  distal  ends 


312         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

grow  outward  until  they  are  just  beneath  the  ectoderm,  when  they 
bend  toward  the  posterior  end  of  the  body,  the  anterior  tubules  fusing 
with  those  behind.  From  the  junction  a  tube,  the  pronephric  or 
archinephric  duct,  gradually  grows  backward  just  beneath  the 
ectoderm  (figs.  317,  318)  until  it  reaches  the  posterior  end  of  the  meta- 
coele,  when  it  fuses  with  the  hinder  end  of  the  digestive  tract  (cloaca) 
or  with  the  ectoderm  in  the  vicinity  of  the  anus.  An  opening  now 
breaks  through,  thus  putting  the  coelom  indirectly  in  communication 
with  the  outer  world. 

At  first  the  pronephric  duct  lies  closely  below  the  ectoderm  and  is 
almost  equally  near  the  lining  of  the  metacoele.  As  the  myotomes 
grow  downward  they  come  to  lie  between  the  ducts  and  the  ectoderm 
so  that  eventually  the  ducts  are  just  beneath  the  lining  of  the  definitive 
body  cavity. 

There  has  been  considerable  dispute  as  to  the  origin  of  the  cells  which  form  the 
pronephric  duct.  They  were  long  thought  to  be  solely  of  mesothelial  character, 
arising  by  proliferation  from  the  tube  itself.  Then  it  was  noticed  that  the  back- 
ward-growing tube  fused  at  its  tip  with  the  ectoderm  and  it  was  thought  that  there 
was  an  actual  contribution  of  ectodermal  cells  at  this  point.  This  view  received 
considerable  support  from  its  agreement  with  certain  theoretical  views.  The 
matter  is  not  yet  decided.  The  writer  is  convinced,  from  the  study  of  perfectly 
preserved  material  in  which  cell  boundaries  are  clearly  shown,  that  in  the  sharks 
(Acanthias)  which  were  thought  most  strongly  to  support  the  view  of  ectodermal 
contribution,  that  the  whole  duct  is  of  mesothelial  origin. 

In  the  teleosts  the  dorsal  end  of  the  nephrotome  grows  out  to  form  the  pro- 
nephric tubule,  to  which  both  somatic  and  splanchnic  walls  thus  contribute.  In 
the  amphibia  the  nephrotome  is  not  distinctly  separated  from  the  lateral  plates 
(hypomere)  and  the  pronephric  tubules  are  formed  from  the  common  area. 

The  pronephros  is  functional  for  a  time  in  the  embryos  of  some 
lower  vertebrates;  in  other  groups  it  is  a  rudimentary  and  transitory 
structure,  save  for  its  participation  in  the  oviducts  and  the  ostium 
tubae  abdominale  (see  below).  When  functional  it  takes  the  nitro- 
genous waste  from  the  body  cavity,  while  its  filtering  apparatus  con- 
sists either  of  separate  glomeruli  (one  for  each  tubule)  or  the  glomeruli 
of  the  separate  somites  may  run  together,  forming  a  glomus.  These 
glomeruli  or  the  glomus  of  the  pronephros  do  not  project  into  a  Bow- 
man's capsule,  but  lie  immediately  above  the  dorsal  wall  of  the  coelom^ 
between  the  mesentery  and  the  nephrostomes  (fig.  318),  pushing  the 
epithelium  before  them.  Later,  as  in  the  csecilians,  they  and  the 
nephrostomes  may  be  enclosed  in  a  cavity  cut  off  from  the  coelom, 


UROGENITAL   SYSTEM. 


313 


SO  that  the  whole  resembles  a  renal  corpuscle,  but  is  dijfferent  in  origin. 
In  either  case  the  exuding  fluid  passes  into  the  metacoele  from  which 
it  is  drawn  by  the  cilia  of  the  nephrostomes  and  passed  into  the  tubules. 
The  blood  is  brought  to  the  glomus  or  glomeruli  by  short  segmental 
arteries  arising  from  the  dorsal  aorta  (fig.  318)  and,  after  passing 
through  the  capillaries,  it  is  carried  away  by  the  postcardinal  veins 
of  the  corresponding  side  to  the  heart,  these  veins  keeping  pace  in 
their  backward  development  with  the  development  of  the  nephridial 
tubules. 


Fig.  318. — Stereogram  of  developing  pro-  and  mesonephros.  a,  aorta;  g,  glomus  or 
glomerulus;  m,  mesenter>';  m/,  mesonephric  tubule;  n,  notochord;  nc,  cavity  of  (tit)  nephro- 
tome;  ns,  nephrostome;  pc,  postcardinal  vein;  pd,  pronephric  duct;  pt,  pronephric  tubule; 
ptm,  peritoneal  membrane. 

There  is  much  that  goes  to  show  that  the  pronephros  formerly  had  a  much 
greater  extension  than  at  present,  including  a  larger  number  of  somites.  It  has, 
however,  been  replaced  in  the  adults  of  all  vertebrates  (with  the  possible  exception 
of  Bdellostonm)  by  the  mesonephros,  and  later,  in  the  amniotes,  by  the  metanephros 
as  described  below. 


Mesonephros. — The  mesonephros  or  Wolffian  body  is  the  second 
excretory  organ  to  arise.  It  arises  after  the  pronephros  and  its 
duct  are  formed,  by  the  development  of  a  series  of  mesonephric  tubules, 
which  grow  out  from  the  nephrotomes  behind  those  concerned  in  the 
formation  of  the  pronephros.  These  tubules  extend  laterally  until 
they  meet  and  fuse  with  the  pronephric  duct,  w^hich  now  acts  as  the 
excretory  canal  of  the  new  gland.     In  some  cases  the  point  of  origin 


314  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

of  the  mesonephric  tubules  is  clearly  dorsal  to  that  of  the  pronephric 
tubules  (fig.  318),  and  in  some  cases  (birds,  caecilians)  pro-  and  meso- 
nephric tubules  have  been  described  as  arising  from  the  same  nephro- 
tome,  one  above  the  other.  In  most  ichthyopsida  the  opening  of  the 
nephrotome  into  the  metacoele  forms  the  nephrostome,  but  in  the 
amniotes  this  opening  is  closed  before  the  tubules  are  formed  and 
consequently  nephrostomes  are  lacking  in  the  latter  group. 


Fig.  319. — Stereogram  of  mesonephros.  a,  aorta;  cv,  postcardinal  vein;  g,  genital 
ridge;  gl,  glomerulus;  m,  mesentery;  mc,  Malpighian  corpuscle;  mi,  mesonephric  tubules; 
my,  myotome;  n,  nephrostome;  nc,  notochord;  p,  peritoneal  lining;  w.  Wolffian  duct. 

Segmental  arteries  grow  out  from  the  aorta  to  the  splanchnic  wall 
of  each  nephrotome,  forming  there  a  network  of  capillaries  at  a  higher 
level  than  the  pronephric  glomeruli  (fig.  319).  The  glomerulus  thus 
formed  presses  the  wall  before  it,  while  the  rest  of  the  nephrotome 
closes  around  it  as  a  Bowman's  capsule,  the  whole  forming  a  Mal- 
pighian body  (in  some  rodents  the  glomeruli  are  rudimentary  or  absent). 
In  most  ichthyopsida  the  Malpighian  body  is  connected  on  one  side 
with  the  metacoele  by  the  nephrostome,  and  on  the  other  with  the 
mesonephric  tubule.  ^ 


UROGENITAL   SYSTEM.  315 

Thus  at  first  the  mesonephros  is  a  metameric  structure,  extending 
over  a  much  larger  number  of  somites  than  does  the  pronephros  and 
reaching  nearly  to  the  posterior  limits  of  the  metacoele.  As  the  devel- 
opment of  the  embryo  proceeds  the  number  of  tubules  increases  by 
budding  in  a  manner  not  readily  described  (fig.  320).  These  tubules 
unite  with  the  distal  ends  of  those  first  formed,  so  that  the  distal  part 
of  these  form  collecting  tubules.  Each  of  these  secondary  tubules 
forms  its  own  Malpighian  body  and  all  of  the  tubules  elongate,  be- 
come convoluted,  and  the  mesonephros  loses  its  primitive  metameric 
character. 


a»»;»i^«s^iai^caE^*^3^%s^ai*s*r^««; 


Fig.  320. — Reconstruction  of  three  somites  of  the  Wolfl5an  body  (mesonephros)  of 
Hypogeophis,  after  Brauer.  a,  aorta;  w^-w^,  primary  and  secondary  Malpighian  bodies; 
n^-n^,  corresponding  nephrostomes;  s,  tertiary  segments  of  mesonephros;  t^-t^,  primary 
and  secondary  mesonephric  tubules;  w.  Wolffian  duct. 

At  the  same  time  changes  are  introduced  into  the  mesonephric 
circulation.  The  veins  emerging  from  the  renal  corpuscles  extend 
out  into  the  region  of  the  tubules,  each  breaking  up  there  into  a  second 
system  of  capillaries  which  envelop  the  tubules  before  returning  the 
blood  to  the  postcardinal  vein.  The  subcardinal  vein  (p.  279)  brings 
the  blood  from  the  caudal  region  (and  usually  from  the  hind  limbs) 
to  the  Wolfl&an  body  and  this  is  also  returned  via  the  postcardinals  to 
the  heart.  (For  details  of  the  modifications  of  the  mesonephric  circu- 
lation see  pages  290-292.) 

The  Mesonephric  Ducts. — The  conditions  in  the  elasmobranchs 
have  been  regarded  as  very  primitive.  In  them  (and  to  some  extent 
in  some  of  the  amphibia),  when  the  mesonephros  develops,  the  pro- 
nephric  duct  divides  longitudinally  from  its  hinder  end  as  far  forward 
as  the  anterior  end  of  the  Wolffian  body.  Of  the  two  ducts  thus 
formed  (fig.  321,  A)j  one,  the  Wolffian  (Leydig's)  duct,  remains 
connected  with  the  tubules  of  the  mesonephros  and  forms  its  excretory 
canal.     The  other,  the  MUllerian  duct,  is  similarly  related  to  the 


3l6         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

pronephros  and  its  derivatives,  and  in  the  female  forms  the  tube 
(oviduct)  by  which  the  eggs  are  carried  to  the  exterior.  In  other 
amphibia  and  in  the  amniotes  the  pronephric  duct  does  not  divide, 
but  remains  solely  in  the  service  of  the  mesonephros  and  forms  the 
Wolffian  duct,  while  the  oviduct  arises  in  another  manner,  to  be  de- 
scribed in  connexion  with  the  reproductive  organs.  In  the  teleosts 
also  there  is  no  division  of  the  pronephric  duct. 


Fig.  321. — Diagrams  of  urogenital  structures  in  (.4)  indifferent  and  in  female  elas- 
mobranchs  and  amphibians;  (B)  male  elasmobranchs  and  amphibians;  (C)  male  amniote 
(mammal);  (D)  female  amniote  (mammal),  b,  urinary  bladder;  c,  cloaca;  e,  epididymis; 
k,  kidney  (metanephros);/,  Fallopian  tube;  g,  gonad;  h,  'stalked  hydatid';  /,  longitudinal 
tubule;  m,  MuUerian  duct  (oviduct),  rudimentary  in  B  andC;  mn,  mesonephros;  o,  ovary; 
ot,  ostium  tubae  abdominale;  pd,  paradidymis;  po,  paroophoron;  pv,  parovarium;  r,  rectum; 
t,  testis;  u,  uterus;  ua,  urethra;  ur,  ureter;  va,  vas  aberrans;  vd,  vas  deferens;  ve,  vasa  effer- 
entia;  w,  Wolfl5an  duct,  urinary  in  A,  urogenital  in  B,  genital  in  C  and  rudimentary  in  D. 


Metanephros. — The  mesonephros  is  functional  in  the  embryos  of 
all  vertebrates  and  throughout  life  in  the  ichthyopsida.  It  also  func- 
tions for  a  short  time  after  birth  in  certain  reptiles  (lizards)  and  in  the 
lowest  mammals  {Echidna,  opossum).  It  becomes  replaced  in  the 
adults  of  all  amniotes  by  the  mesonephroi,  the  only  structures  to  which 
the  name  kidneys  is  strictly  applicable.  Each  metanephros  arises 
behind  the  mesonephros  of  the  same  side.  From  the  dorsal  hinder 
end  of  the  Wolffian  duct,  near  its  entrance  into  the  cloaca,  a  tube,  the 


UROGENITAL   SYSTEM. 


317 


Tris 


Fig.  322. — Profile  reconstructions  of  lizard  {Lacerta  agilis)  {A)  16  mm.  long;  (JB)  20  mm. 
long;  and  (C)  human  embr\-o  115  mm.  long,  after  Schreiner.  a,  allantois  stalk;  c,  cloaca; 
cc,  cranial  collecting  tubule ;  cd,  caudal  collecting  tubule ;  k,  permanent  kidney  (metanephros) ; 
met,  median  collecting  tubule;  mt,  metanephric  (nephrogenous)  tisssue;  mtb,  mesonephric 
tubules;  pet,  primary  collecting  tubule;  pii,  Wolffian  duct  (primitive  ureter);  r,  rectimi; 
5,  secondar}-  collecting  tubule;  u,  ureter;  cm,  u  and  pu,  common  portion  of  primitive  and 
permanent  ureters. 


Fig.  323. — Models  of  two  stages  in  the  development  of  tubules  of  kidney  (metanephros) 
of  man,  after  Stoerk.  b,  Bowman's  capsule;  e,  collecting  tubule;  en,  connecting  tubule; 
cv,  convoluted  tubule;  h,  Henle's  loop;  /,  intercalar)'  tubule;  /,  lower  arch;  m,  middle  piece. 


3l8        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

ureter  (fig.  322,  k)  grows  forward,  parallel  to  the  parent  duct,  into 
the  tissue  posterior  and  dorsal  to  the  mesonephros.  This  nephro- 
genous tissue  is  apparently  serially  homologous  with  that  from  which 
the  mesonephric  tubules  have  arisen,  but  all  traces  of  metamerism 
have  disappeared  from  it.  In  this  nephrogenous  tissue  the  anterior 
end  of  the  ureter  gives  off  a  varying  number  of  branches  (fig.  322), 
each  of  which  expands  at  its  tip,  thus  forming  a  primary  renal  vesicle, 
and  a  little  later  the  place  where  the  branches  and  the  ureter  unite 
expands,  the  enlargement  forming  the  pelvis  of  the  definitive  kidney. 
The  cells  of  the  nephrogenous  tissue  form  a  number  of  aggregates 
around  each  primary  vesicle;  each  aggregate  soon  becomes  hollow, 
and  develops  into  an  S-shaped  tubule  (fig.  323,  left),  one  end  of  which 
joins  the  primary  renal  vesicle,  while  a  glomerulus  arises  at  the  other 
end,  but  no  nephrostomes  are  formed.  Later  there  is  a  great  mul- 
tiplication of  these  tubules  and  an  extension  of  the  capillary  system 
of  the  glomeruli  around  them,  much  as  in  the  mesonephros.  The 
differentiation  of  each  tubule  into  convoluted,  collecting  and  Henle's 
regions  occurs  early  (fig.  323,  right). 

Urinary  Bladder. — At  or  near  the  hinder  ends  of  the  excretory 
ducts  there  is  frequently  a  reservoir  for  the  urine,  the  urinary  bladder 
or  urocyst.  Of  these  there  may  be  three  kinds.  In  most  fishes  the 
bladder  arises  by  a  fusion  of  the  hinder  ends  of  the  Wolffian  ducts 
plus  a  part  derived  from  the  hinder  end  of  the  digestive  tract  (cloaca), 
the  Wolffian  ducts  emptying  into  it  and  the  whole  opening  to  the 
exterior,  usually  dorsal  and  posterior  to  the  anus.  In  the  dipnoi  there 
is  a  diverticulum  from  the  dorsal  wall  of  the  cloaca,  anterior  to  the 
openings  of  the  Wolffian  ducts.  This  is  usually  called  the  urinary 
bladder  (fig.  325,  Z)),  but  it  may  be  homologous  with  the  rectal  gland 
of  the  elasmobranchs. 

The  third  type,  the  allantoic  bladder,  occurs  in  all  tetrapoda. 
This  arises  as  a  ventral  diverticulum  from  the  cloaca.  In  the  amphibia 
the  whole  of  the  outgrowth  forms  the  bladder  and  its  walls  are  sup- 
plied by  the  hypogastric  arteries.  In  the  amniotes  the  proximal 
portion  alone  is  converted  into  the  urinary  bladder,  while  the  more 
distal  portion,  in  the  embryo  becomes  the  respiratory  organ  of  the 
growing  young,  the  allantois.  This  part  extends  far  beyond  the  body 
wall,  carrying  with  it  branches  of  the  hypogastric  arteries  (allantoic 
arteries),  and  in  the  mammals  forms  a  part  of  the  placenta.  The 
allantois  becomes  reduced  in  the  later  stages  and  at  the  beginning  of 


UROGENITAL   SYSTEM.  319 

free  life  is  entirely  absorbed  or  is  lost  with  the  placenta.  In  the 
amphibia  the  urine  finds  its  way  into  the  urinary  bladder  via  the  cloaca, 
as  the  urinary  ducts  (Wolffian  ducts)  do  not  open  into  it.  In  those 
amniotes  in  which  a  bladder  is  present  the  ureters  open  into  it,  and 
the  urine  is  conveyed  to  the  exterior  by  a  single  tube,  the  urethra. 
In  many  sauropsida  there  is  no  urinary  bladder,  though  the  allantois 
is  formed  in  development. 

There  is  great  dijficulty  in  comparing  the  excretory  system  of  the  vertebrates 
with  anything  known  in  the  invertebrates.  In  general  the  nephridial  tubules  may  ' 
be  compared  with  those  of  the  annelids.  Both  have  nephrostomes  opening  into 
the  coelom,  convoluted  tubules,  enveloped  in  a  network  of  capillary  blood-vessels, 
but  in  the  annelid  each  tubule  opens  separately  to  the  exterior  in  the  somite  behind 
that  in  which  the  nephrostome  lies,  while  in  the  vertebrate  the  series  of  tubules 
empty  into  a  common  duct.  When  it  was  thought  (p  312)  that  the  ectoderm  con- 
tributed to  the  pronephric  duct,  the  homologies  appeared  easy.  The  duct  was 
originally  a  groove  on  the  outer  surface  into  which  the  separate  tubules  opened. 
Then  the  groove  was  rolled  into  a  tube  which  continued  backward  to  the  vicinity 
of  the  anus  By  the  downgrowth  of  the  myotomes  the  duct  became  cut  off  from 
its  primitive  position  and  came  to  lie  just  outside  the  peritoneal  lining  When, 
however,  it  is  considered  that  in  all  probability  the  pronephric  duct  is  formed  solely 
from  the  mesoderm  the  homology  falls  to  the  ground  and  an  explanation  is  still  a 
desideratum. 

THE  REPRODUCTIVE  ORGANS. 

The  tissue  which  is  to  form  the  ovaries  and  testes  early  forms  a 
pair  of  genital  ridges,  one  on  either  side  of  the  mesentery  and  between 
it  and  the  Wolffian  ridge  (fig.  319).  At  one  time  it  was  thought  that 
the  anlage  of  the  gonad  was  segmental  in  character  and  '  gonotomes,' 
comparable  to  nephro tomes  and  myotomes,  were  described.  It  has 
since  been  shown  that  no  metamerism  exists  and  that  the  primary 
germ  cells,  which  alone  characterize  the  gonads,  arise  in  several  groups 
of  vertebrates  (possibly  in  all)  from  the  entoderm,  which  is  never 
metameric.  At  about  the  time  of  the  differentiation  of  the  somites 
they  migrate  through  the  developing  mesoderm  to  their  definitive  posi- 
tion in  the  epithelium  of  the  genital  ridges,  the  primitive  or  primordial 
ova  (whether  to  form  eggs  or  sperm)  being  recognizable  from  their 
size  and  their  reaction  to  microscopic  stains  (fig.  324,  0).  In  the 
adults  of  many  vertebrates  the  gonads  at  maturity  project  far  into 
the  coelom  and  are  often  suspended  by  a  fold  of  peritoneum  which  is 
called  a  mesorchitmi  in  the  male,  a  mesoarium  in  the  female. 


320         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

Ovaries. — In  the  ovarian  epithelium  the  primitive  ova  multiply, 
and  the  products,  accompanied  by  some  of  the  epithelial  cells,  sink 
into  the  deeper  stroma  of  connective  tissue,  thus  forming  ovarial  cords 
each  containing  a  number  of  ova.  Then  the  cords  break  up  and  each 
egg  becomes  surrounded  with  a  layer  of  epithelial  cells,  the  whole 
forming  a  Graafian  follicle,  the  follicle  cells  supplying  nourishment 
to  the  contained  ovum.  In  the  higher  vertebrates  there  is  a  great 
increase  in  the  number  of  follicle  cells,  which  become  arranged  in 
several  layers.     Then  a  split  arises  in  the  follicle,  the  cavity  becoming 


Fig.  324. — Section  of  genital  ridge  of  chick  of  five  days'  incubation,  after  Semon.    e,  epithel- 
ium of  ridge  (coelomic  wall) ;  c,  medullary  cords;  0,  primordial  ova. 

filled  with  a  follicular  liquor,  while  the  ovum,  surrounded  by  several 
layers  of  cells,  adheres  to  one  side  of  the  cavity,  this  part  being  called 
the  discus  proligerus. 

When  the  eggs  have  attained  their  full  size  and  the  proper  time 
has  arrived  some  of  the  follicles  migrate  to  the  surface  of  the  ovary, 
then  the  follicles  rupture  and  the  contained  ova  escape  into  the  coelom. 
Their  history  from  this  point  will  be  outlined  in  connection  with  the 
genital  ducts.  Each  ruptured  follicle  (at  least  in  elasmobranchs, 
amphibians  and  amniotes  leaves  a  scar  on  the  surface  of  the  ovary — 
the  corpus  luteum — characterized  by  the  presence  of  peculiar  ('  lutein ') 
cells. 

Testes. — In  the  gonads  of  the  male  (testes)  there  is  a  somewhat 
similar  insinking  of  the  primordial  ova  and  epithelial  cells  into  the 
stroma  of  the  genital  ridge,  but,  instead  of  breaking  up  into  separate 
follicles,  each  sexual  cord  develops  a  lumen  and  becomes  converted 


UROGENITAL   SYSTEM.  32 1 

into  a  seminiferous  tubule,  in  the  walls  of  which  both  the  epithelial 
cells  and  the  primordial  ova  are  recognizable,  as  well  as  a  third  kind 
of  cell,  called  Sertoli's  cell,  concerning  which  accounts  are  some- 
what at  variance,  some  regarding  them  as  derivatives  of  the  epithelial 
cells,  others  as  coming  from  the  primitive  germ  cells.  They  play  no 
part  in  the  actual  formation  of  the  spermatozoa,  but  act  rather  as 
nutritive  or  'nurse  cells'  for  the  developing  spermatozoa.  For  the 
differentiation  of  the  germ  cells  into  spermatozoa  reference  must  be 
made  to  the  text-books  of  embryology  and  histology.  In  most  verte- 
brates the  testes  continue  in  the  position  where  they  first  appear,  but 
in  most  mammals  they  eventually  descend  to  a  position  outside  of  the 
body  cavity  and  are  enclosed  in  a  special  pouch,  the  scrotum.  This 
descent  of  the  testes  is  described  in  connection  with  the  reproductive 
organs  of  the  mammals,  below. 

THE  REPRODUCTIVE  DUCTS. 

The  reproductive  products  formed  in  the  gonads  have  to  be  carried 
to  the  exterior,  either  as  spermatozoa,  or  as  eggs  or  young  in  different 
stages  of  development,  the  ducts  in  the  male  being  called  vasa  def  er- 
entia,  those  of  the  female  being  oviducts.  The  former  are  usually 
the  Wolffian  ducts,  the  latter  may  be  either  the  Mullerian  ducts  or 
tubes  developed  for  the  special  purpose,  or  lastly,  the  abdominal  pores. 

Male  Ducts. — In  elasmobranchs,  amphibia  and  amniotes  the 
Wolffian  ducts  (fig.  321)  serve  as  the  outlet  for  the  sperm.  While 
the  seminiferous  tubules  are  developing,  there  occurs  a  proliferation 
of  cells  from  the  wall  of  the  Bowman's  capsules  in  the  anterior  end 
of  the  mesonephros.  These  medullary  cords  extend  through  the 
adjacent  connective  tissue  and  into  the  genital  ridge  where  they  come 
into  close  connexion  with  the  developing  seminiferous  tubules  (fig. 
324).  When  the  latter  acquire  their  lumen  the  medullary  cords  also 
become  canalized,  so  that  both  form  a  continuous  transverse  tubule 
(vas  efferens)  leading  from  the  genital  cells  to  the  Malpighian  cor- 
puscles, and  thence  by  the  mesonephric  tubules  to  the  Wolfl&an  duct 
(fig.  325,  .4).  These  vasa  efferentia  become  connected  by  a  longi- 
tudinal canal  before  entering  the  Wolfl&an  body,  while  usually  there 
is  another  longitudinal  canal  connecting  them  in  the  body  of  the  testis 
(fig.  321,  B).  Usually  this  connexion  of  testis  and  Wolfl&an  body 
takes  place  at  the  anterior  end  of  the  mesonephros,  but  in  some  dipnoi 


322  COMPARATIVE   MORPHOLOGY   OF   VERTEBRATES. 

the  posterior  end  of  the  mesonephros  alone  is  involved.  This  is  fre- 
quently accompanied  by  a  degeneration  of  the  glomeruli  of  the  tubules 
concerned,  so  that  this  part  of  the  mesonephros  loses  its  excretory 
character  and  becomes  subsidiary  to  reproduction.  With  this  forma- 
tion of  vasa  efferentia  the  sperm  never  enters  the  coelom  except  as  this 
is  represented  in  the  cavities  of  the  mesonephric  tubules. 

As  a  farther  result  the  anterior  end  of  the  Wolffian  duct  becomes 
purely  reproductive  in  the  male  and  is  usually  greatly  coiled,  this 
portion  being  called  the  epididymis.  In  the  amniotes,  where  the 
hinder  portion  of  the  mesonephros  is  supplanted  by  the  true  kidney 
(metanephros) ,  the  whole  Wolffian  duct  is  a  sperm  duct  (vas  deferens) 
in  the  male,  while  in  the  female  it  largely  or  completely  degenerates. 
In  the  amphibia  and  elasmobranchs  the  hinder  end  of  the  duct  is  both 
reproductive  and  excretory  in  the  male;  in  the  female  it  is  purely 
excretory. 

In  the  ichthyopsida,  other  than  elasmobranchs  and  amphibia,  the 
sperm  is  carried  to  the  exterior  in  other  ways,  and  there  is  no  connexion 
of  the  testes  with  the  excretory  organs.  In  the  cyclostomes  the  sperm 
escapes  from  the  testes  into  the  coelom  and  then  is  passed  to  the  exterior 
by  way  of  the  abdominal  pores  (p.  124)  which  in  the  lampreys  open 
into  a  cavity  (sinus  urogenitalis)  which  also  receives  the  hinder  ends 
of  the  Wolffian  ducts.  In  the  myxinoids  the  pores  are  united  and 
open  to  the  exterior  behind  the  anus  and  between  it  and  the  urinary 
openings. 

The  conditions  found  in  the  sturgeons  (fig.  325,  A)  and  in  Polyp- 
terus  give  a  possible  explanation  to  the  aberrant  structures  of  the  tele- 
osts.  In  the  first  group  can  be  made  out  the  vasa  efferentia  and  the 
two  longitudinal  canals  connecting  them,  these  extending  the  whole 
length  of  the  testis.  In  Polypterus  (fig.  325,  C)  the  connexion  between 
the  testis  and  mesonephros  is  confined  to  the  hinder  portion  of 
organs,  the  anterior  vasa  efferentia  and  the  longitudinal  canal 
disappearing  in  front,  the  longitudinal  testicular  canal  taking  the 
sperm  from  the  anterior  end  of  the  testis  and  carrying  it  farther  back 
for  passage  through  the  mesonephros.  Here  the  anterior  end  of  the 
Wolffian  duct  is  purely  excretory.  A  farther  concentration  of  the 
efferent  functions  to  the  last  vas  efferens  would  give,  with  a  few  other 
modifications,  the  conditions  of  the  teleosts  (fig.  325,  -S).  In  all  of 
this  group  there  is  no  connexion  of  testes  with  mesonephroi.  The 
seminiferous  tubules  are  connected  by  a  longitudinal  canal  (apparently 


UROGENITAL   SYSTEM. 


323 


the  longitudinal  testicular  canal  of  other  vertebrates)  which  runs  in 
the  membrane  (mesorchium)  supporting  the  testis,  back  to  the  external 
opening,  which  is  either  directly  to  the  exterior  between  the  urinary 
opening  and  the  anus  (fig.  328)  or  into  a  urogenital  sinus  (fig.  321, 5). 

This  view  is  farther  supported  by  the  relations  in  the  dipnoi.  In 
Ceratodus  there  are  numerous  vasa  efferentia  which  extend  from  the 
testis  into  the  mesonephros.     In  Lepidosiren  the  efferent  ductules  are 


Fig.  325. — Diagrams  of  urogenital  organs  of  male  fishes,  after  Goodrich.  Ay  Acipenser 
(Lepidosteus  and  Amia  similar,  but  lack  the  oviduct);  B,  teleosts;  C,  Polypterus;  D,  Pro- 
topterus;  E,  urogenital  openings  of  female  salmon,  a,  anus;  ap,  abdominal  pore;  c6, 
cloacal  ('urinary')  bladder;  e,  vasa  efferentia;  gp,  genital  pore  (papilla);  m,  mesonephros; 
md,  Mullerian(?)  duct;  r,  rectum;  re,  renal  corpuscle;  s,  urogenital  sinus;  t,  testis;  u,  up, 
urinary  pore;  ugp,  urogenital  pore;  v,  vas  deferens;  w,  Wolfl&an  duct. 


fewer  in  number  and  they  arise  from  a  posterior  degenerate  portion 
of  the  testis,  while  in  Protopterus  (fig.  325,  D)  there  is  but  a  single 
vas  efferens  on  either  side  and  this  passes  through  the  posterior  end 
of  the  Wolffian  body. 

Oviducts. — In  the  elasmobranchs  the  Miillerian  duct,  which,  as 
described  above,  arises  by  a  splitting  of  the  pronephric  duct,  serves 
as  the  oviduct.  After  separation  from  the  Wolffian  duct  this  opens 
in  front  into  the  coelom  by  means  of  the  pronephric  tubules  and  their 


324        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

nephrostomes.  Then  these  flow  together,  forming  a  large  opening, 
the  ostium  tubae  abdominale,  on  either  side  (in  elasmobranchs  the 
ostia  of  the  two  sides  are  usually  united  ventral  to  the  liver)  through 
which  the  eggs,  which  pass  from  the  ovary  into  the  coelom  are  carried 
into  the  oviduct. 

In  some  amphibia  (Salamandra)  the  pronephric  tubules  and  neph- 
rostomes take  a  part  in  the  formation  of  the  ostium  tubae  and  the 
beginning  of  the  oviduct,  while  in  Amhly stoma  the  ostium  develops 
in  close  connection  with  the  pronephric  nephrostomes.  Here,  as  in 
all  other  tetrapoda,  the  rest  of  the  oviduct  arises  by  the  formation  of  a 
groove  of  the  peritoneal  membrane  close  beside  the  Wolffian  duct. 
This  becomes  rolled  into  a  tube,  the  Mullerian  duct.  In  the  amniotes 
the  anterior  end  of  the  groove  does  not  close,  but  remains  open  as  the 
ostium  tubae  (fig.  321,  ^). 

Usually  the  condition  in  the  elasmobranchs  has  been  regarded  as 
the  primitive  one,  a  supposition  which  renders  it  difficult  to  homologize 
the  Mullerian  ducts  (oviducts)  of  elasmobranchs  with  those  of  other 
forms.  Still,  when  the  adult  conditions  are  considered — similar  ostia, 
similarity  of  position,  and  of  external  openings — it  is  hardly  possible 
to  believe  them  as  merely  analogous,  as  examples  of  convergence. 
The  facts  in  the  amphibia,  referred  to  in  the  preceding  paragraph 
are  additional  evidence  of  homology.  If,  however,  it  be  assumed 
that  the  more  common  type  of  development,  by  the  infolding  of  coe- 
lomic  epithelium,  be  the  primitive  condition,  the  difficulties  are  less, 
though  not  entirely  solved.  Then,  if  it  be  that  the  homologous  tissue 
in  the  elasmobranchs  was  at  first  included  in  the  tissue  of  the  pro- 
nephric duct  and  that  the  splitting  is  a  secondary  operation  to  separate 
parts  which  elsewhere  are  always  distinct,  the  similarities  are  more 
apparent. 

In  the  females,  as  in  the  males,  of  cyclostomes  and  teleosts  the 
reproductive  ducts  are  not  easily  brought  into  harmony  with  those 
of  other  vertebrates,  and  an  answer  to  all  questions  cannot  be  had  until 
the  development  of  the  parts  has  been  studied  in  more  forms,  and 
especially  the  ganoids  and  dipnoi.  In  the  cyclostomes  the  eggs  are 
shed  from  the  ovaries  into  the  coelom  and  are  thence  passed  outward 
through  the  abdominal  pores. 

In  the  teleosts  there  are  several  conditions.  The  ovaries  may  be 
simple  and  solid  bands  or  saccular  in  character  with  an  internal  lumen 
(fig.  326,  £).     In  the  first  the  eggs  pass  into  the  coelom  and  thence 


UROGENITAL   SYSTEM. 


325 


to  the  exterior  by  abdominal  pores  or  by  oviducts  of  varying  lengths 
(fig.  326,  F).  Concerning  the  nature  of  these  ducts  there  is  uncer- 
tainty. They  may  be  true  Miillerian  ducts  or  new  formations  within 
the  group.  The  fact  that  similar  tubes  occur,  with  permanently  open 
ostia  in  both  sexes  of  the  sturgeons  (fig.  325),  and  that  these  open 


Fig,  326. — Diagrams  of  urogenital  organs  of  female  fishes,  after  Goodrich.  A,  Pro- 
topterus;  B,  Polypterus;  C,  Amia;  D,  Lepidosteus;  E,  most  teleosts;  F,  salmonid.  ap,  ab- 
dominal pore;  c6,  cloacal  bladder;  d,  cloaca;/,  funnel  of  oviduct;  gp,  genital  pore  or  papilla; 
w,  mesonephros;  o,  ovary;  od,  oviduct;  r,  rectum;  s,  urogenital  sinus;  up,  urinary  pore, 
(papilla) ;  ugp,  urogenital  pore  (papilla) ;  w,  Wolffian  ducts. 

behind  into  the  Wolffian  ducts,  lends  probability  to  the  view  that  the 
ducts  of  the  ordinary  teleosts  are  Miillerian  in  character,  but  greatly 
modified. 

The  saccular  condition  of  the  ovaries  appears  to  arise  in  two  ways. 
In  the  one  the  primitively  free  edge  of  the  ovary  bends  laterally  and 
fuses  with  the  coelomic  wall,  thus  enclosing  a  cavity,  the  parovarial 


326        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

canal,  closed  in  front.  In  the  other  type  a  groove  of  the  covering 
epithelium  forms  on  the  surface  of  the  ovary.  This  closes  over  and 
sinks  inward,  forming  what  is  termed  as  an  entovarial  canal.  Either 
canal  may  extend  backward  to  the  hinder  end  of  the  body  cavity,  thus 
forming  an  oviduct,  or  the  oviduct  may  be  formed  from  both  kinds  of 
canals,  one  in  front,  the  other  behind.  From  this  it  would  appear 
that  the  ovary  originally  extended  back  to  the  hinder  end  of  the  ccelom 
(as  it  does  in  Cyclopterus)  or  that  the  par-  or  entovarial  canal  had 
united  with  a  Miillerian  duct  which  has  otherwise  been  entirely  lost. 
The  oviducts  thus  formed  usually  unite  before  opening  to  the  exterior, 
either  directly  ox  via  a  urogenital  sinus.  The  oviducts  in  the  dipnoi 
(fig.  326,  A)  are  much  like  those  of  the  selachians,  emptying  inde- 
pendently into  the  cloaca.  They  persist,  though  of  small  size,  in  the 
males  (fig.  325,  Z)). 

EXCRETORY  ORGANS  IN  THE  SEPARATE  GROUPS. 

CYCLOSTOMES. — ^In  the  lampreys  the  pronephros  extends  over  thirteen 
'  somites,  but  only  the  anterior  five  form  complete  tubules,  the  remainder,  however, 
join  the  pronephric  duct.  The  pronephros  is  best  developed  in  the  Ammocoete, 
10  mm.  long,  and  in  this  stage  the  mesonephros  is  also  developed  and  both  are 
functional.  With  increase  in  size  there  is  a  degeneration  of  the  mesonephric  tubules 
in  front  and  a  formation  of  new  ones  behind,  the  definitive  organ  extending  over 
about  two-fifths  of  the  body  length.  Each  pronephros  projects  into  the  ccelom  as 
a  band  supported  by  a  fold  of  the  peritoneal  membrane.  The  two  pronephric  ducts 
unite  a  little  in  front  of  the  hinder  end,  forming  a  urogenital  sinus  into  which  the 
abdominal  pores  empty,  and  which,  in  turn,  opens  at  the  tip  of  a  urogenital  papilla 
just  behind  the  anus. 

In  the  myxinoids  the  nephridial  tubules  develop  as  a  continuous  series,  the 
organ  in  the  earliest  stage  known  extending  over  somites  11-80.  Later  the  organ 
becomes  divided  into  two  parts  by  the  degeneration  of  the  intermediate  tubules. 
The  anterior  part  projects  into  the  body  cavity  and  is  provided  with  nephrostomes, 
while  the  posterior  part,  reaching  through  some  twenty  or  thirty  somites,  has  its 
tubules  strictly  segmental,  each  with  a  Malphigian  body.  This  is  the  functional 
excretory  organ. 

ELASMOBRANCHS. — ^The  pronephros  is  never  functional  as  an  excretory 
organ.  The  Wolffian  bodies  of  the  two  sides  are  somewhat  influenced  in  form  by  the 
other  viscera,  and  are  sometimes  asymmetrical.  Usually  the  nephrostomes  are  closed 
in  the  adult,  but  they  persist  in  several  genera,  among  them  Acanthias,  while  they  are 
lacking  in  Scyllium  and  Rata.  The  anterior  end  of  each  mesonephros  is  narrowed 
and  serves  as  the  connexion  with  the  testes  in  the  male,  while  the  anterior  end  of 
the  Wolffian  duct  forms  a  much-coiled  epidymis  in  the  same  sex.  A  urinary  blad- 
der is  formed  by  the  union  of  the  ducts  of  the  two  sides.     In  the  female  the  blad- 


UROGENITAL   SYSTEM.  327 

der  opens  to  the  exterior  at  the  tip  of  a  genital  papilla,  but  in  the  male  it  connects 
with  a  urogenital  sinus,  into  which  a  pair  of  reservoirs  of  sperm  empty.  The  duct 
from  the  urogenital  sinus  opens  into  the  cloaca  at  the  tip  of  a  urogenital  papilla. 
In  Chimara  the  anterior  end  of  the  mesonephros  lacks  Malphigian  bodies  and  forms 
a  large  (Leydig's)  gland,  the  secretion  of  which  may  possibly  be  used  in  dissolving 
the  spermatophores  (fig.  331). 

GANOIDS. — In  Polypterus  the  pronephric  tubules  are  two  in  number,  belonging 
to  the  second  and  fifth  post-otic  somites;  in Lepidosteus  there  are  five  or  six;  sturgeon 
six;  and  Amia  eight  to  eleven.  The  large  size  of  the  pronephros  in  Polypterus  is 
due  to  the  extensive  coiling  of  the  anterior  end  of  the  duct.  In  the  sturgeon  a  part 
of  the  excretory  organ  is  separated  from  the  rest  but  it  is  not  certain  that  this  is 
really  a  pronephros. 

The  mesonephros  is  markedly  segmental,  the  glands  of  the  two  sides  being  en- 
larged and  united  behind  in  the  sturgeon.  Nephrostomes  are  late  in  appearance,  not 
being  formed  until  after  the  tubules  have  joined  the  duct.  The  urinary  bladder 
differs  from  that  of  teleosts  in  that  the  Miillerian  ducts  enter  it. 

TELEOSTS  (fig.  327)  have  a  pronephros  which  extends  over  from  one  to  five 
somites.  It  is  usually  transitory  in  character,  but  it  persists  through  life  in  several 
species  and  functions  during  the  larval  stages  in  many  more.  The  mesonephros 
varies  considerably  in  shape.  Where  there  is  an  air  bladder  this  covers  some  or  all 
of  the  ventral  surface  of  the  mesonephroi.  Frequently  the  organs  of  the  two  sides 
are  united  behind,  while  lobes  may  extend  forward  from  the  main  mass,  or  back 
into  the  tail.  The  duct  is  sometimes  visible  from  below,  sometimes  it  is  immersed 
in  the  mass  of  the  organ.  There  is  no  sexual  part  to  the  mesonephros  and  there 
are  no  nephrostomes  in  the  adult.  The  urinary  ducts  of  the  two  sides  unite  behind 
and  from  the  united  portion  and  from  the  ventral  wall  of  the  cloaca  the  urinary 
bladder  is  formed.  Later  the  opening  of  the  bladder  separates  from  the  cloaca  and 
usually  comes  to  lie  behind  the  anus,  sometimes  united  with  the  sexual  openings. 

DIPNOI. — In  Ceratodus  there  are  two  pronephric  tubules,  that  of  the  third 
somite  being  complete,  that  of  the  fourth  rudimentary.  The  glomerulus  lies  beside 
the  open  nephrostome.  The  mesonephros  is  at  first  strongly  metameric.  There 
are  no  nephrostomes  in  the  adult  and  none  appear  at  any  time  in  Lepidosiren.  The 
adult  mesonephros  is  widest  behind,  but  the  relations  of  the  efferent  ductules  of  the 
male  are  differently  arranged  in  the  separate  genera,  as  mentioned  above. 

AMPHIBIA. — The  pronephros  (developing  from  two  somites  in  the  urodeles, 
three  in  anura  and  twelve  or  more  in  gymnophiones)  retains  its  functions  in  uro- 
deles and  anura  until  the  metamorphosis,  when  its  tubules  degenerate.  At  first 
the  mesonephros  consists  of  a  tubule  with  nephrostome  and  renal  corpuscle  for  each 
somite,  but  in  the  adult  this  metamerism  is  lost,  except  at  the  anterior  end,  by  the 
development  of  secondary  tubules,  each  complete  like  the  original  ones,  the  nephro- 
stomes sometimes  amounting  to  over  a  thousand  on  the  ventral  surface  of  each 
Wolffian  body.  In  the  adult  anura  the  nephrostomes  lose  their  connexion  with 
the  excretory  system  and  join  branches  of  the  renal  arteries,  thus  placing  the  coelom 
in  connexion  with  the  circulatory  system.. 

In  the  urodeles  the  mesonephroi  form  a  pair  of  ridges  on  the  dorsal  wall  of  the 
coelom,  but  they  occasionally  project  as  folds.     Their  length  is  somewhat  propor- 


328 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


tional  to  the  total  body  length.  The  anterior  end  of  each  loses  its  excretory  char- 
acter and  in  the  male  becomes  accessory  to  reproduction,  as  described  above  (p. 
522).  In  the  anura  the  organs  are  more  compact  and  the  differentiated  anterior 
end  is  lacking,  though  the  efferent  ductules  of  the  testes  pass  through  the  organ. 
The  caecilians  (fig.  334)  resemble  the  urodeles,  except  in  having  the  mesonephroi 
more  lobulated,  the  result  of  aggregates  of  tubules  around  the  collecting  tubules. 


pes 


pcd 


Fig.  327 . — Urinary  organs  of  teleosts,  after  Haller.  A,  pronephros  and  ducts  of  young 
Salmo  fario;  B,  excretory  organs  of  adult  perch,  Percafiuviatilis;  C,  of  carp,  Cyprinus  carpio; 
a,  aorta;  cv,  caudal  vein;  d,  urinary  duct;  m,  mn,  mesonephros;  pcd,  pes,  right  and  left 
postcardinal  veins;  p,  pn,  pronephros;  r,  rectum;  u,  urinary  bladder;  w,  ivd,  Wolffian  duct. 


The  Wolffian  ducts  are  excretory  in  both  sexes  and  are  also  reproductive  in  the 
male.  The  ducts  of  the  two  sides  open  separately  into  the  cloaca,  with,  usually  in 
the  male,  an  enlargement,  the  seminal  vesicle,  which  in  the  breeding  season  serves 
as  a  reservoir  for  the  sperm.  The  urinary  bladder  differs  from  that  of  the  ichthy- 
opsida  in  being  ventral  to  the  cloaca;  it  is  of  the  allantoic  type  (p.  318).     It  is  very 


UROGENITAL   SYSTEM. 


329 


long  in  the  caecilians  (fig.  334)  and  Amphiuma,  saccular  in  most  urodeles,  and  bifid 
at  the  tip  in  most  anura,  being  even  divided  into  two  sacs,  connected  only  at  the 
opening  into  the  cloaca  in  some  species. 

SAUROPSIDA. — In  reptiles  and  birds,  as  in  all  amniotes,  the  pronephros  is 
rudimentary  at  all  stages  and  never  functions  as 
an  excretory  organ.  The  mesonephros  takes  its 
place  in  fcetal  life,  and  in  some  it  continues  to 
function  for  some  time  after  hatching,  but  in  all  it 
is  eventually  replaced  by  the  metanephros,  though 
its  degenerate  remains  persist  in  the  reptiles  (better 
preserved  in  the  female)  forming  the  so-called 
'golden  yellow  body.'  Another  part  is  retained 
in  the  male  as  a  part  of  the  efferent  ductules  of 
the  testes,  somewhat  as  in  mammals. 

The  metanephros  (fig.  328)  never  has  the  ex- 
tent of  the  mesonephros  of  the  ichthyopsida,  but 
it  is  usually  restricted  to  the  posterior  half  of  the 
body  cavdty,  often  to  the  pelvic  region.  It  is  usu- 
ally small  and  compact  (snakes  form  an  exception) 
or  somewhat  lobulated,  in  the  snakes  the  lobulation 
sometimes  being  so  extensive  that  the  lobules  are 
only  connected  by  the  ureter.     In  the  lizards  the 


sS?P^ 


Fig. 


Fig.  329. 


Fig.  328. — Urogenital  organs  of  Monitor,  after  Gegenbaur.  d,  opening  of  digestive 
tract  into  cloaca;  e,  epididymis;  k,  kidney;  />,  papillae  of  urogenital  system;  r,  rectum; 
t,  testes;  u,  ureter;  vd,  vas  deferens. 

Fig.  329. — Urogenital  organs  in  pig  embryo  67  mm.  long,  after  Klaatsch.  a,  allantois; 
g,  gonad;  ms,  mt,  meso-  and  metanephroi;  sr,  adrenal. 


organs  of  the  two  sides  may  be  connected  behind.  In  the  birds  there  are  usually 
three  lobes  in  each  mesonephros,  these  lying  in  cavities  in  the  pelvis  between 
the  sacral  vertebrae  and  the  transverse  processes. 


330 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  Wolffian  ducts  persist  only  as  the  ducts  of  the  testes  (vasa  deferentia)  and 
the  ureters  take  their  place  as  carriers  of  the  nitrogenous  waste.  These  latter 
tubes  open  separately  into  the  cloaca.  An  (allantoic)  urinary  bladder  is  found 
only  in  lizards  and  turtles  (fig.  313).  The  urine  is  semisolid  and  consists  largely 
of  uric  acid. 


MAMMALS. — In  the  mammals  but  two  tubules  are  outlined  in  the 
pronephros  and  these  never  become  functional.  The  pronephric  duct 
is  formed  as  a  solid  cord  on  the  surface  of  the  nephrotomic  segments 
which  later  becomes  canalized.  Of  the  fate  of  the  pronephros 
nothing  certain  is  known.     The  mesonephros  (fig.  329),  on  the  other 

hand,  is  an  important  structure  in  foetal 
life,  and  in  the  monotremes  and  mar- 
supials it  continues  to  function  in  the  im- 
mature stages.  Later  it  largely  disap- 
pears in  all,  with  the  exception  of  the 
parts  concerned  in  the  formation  of  the 
efferent  ductules  of  the  testes  and  some 
inconsiderable  remnants  in  both  sexes. 
Only  in  Echidna  are  nephrostomes 
formed  and  in  some  rodents  there  is  no 
formation  of  glomeruli. 

The  peculiar  development  of  the 
mammalian  metanephros  (p.  316)  results 
in  the  kidney  of  the  young  stages  having 
a  lobulated  appearance,  the  lobules  cor- 
responding to  the  ducts  given  off  from 
the  end  of  the  ureter,  so  that  each  has 
its  own  duct.  This  condition  is  retained 
in  the  adult  elephants,  some  ungulates, 
carnivores  (fig.  330)  and  primates,  and  especially  in  the  aquatic 
species  (whales,  seals),  the  lobules  being  most  numerous  in  some  of 
the  whales.  In  all  other  forms  the  ducts  fuse  later  and  the  lobules 
unite  into  a  compact  mass  lying  in  the  lumbar  region  near  the  last 
rib.  Each  kidney  has  a  peculiar  shape  (giving  rise  to  the  adjec 
tive  reniform),  convex  on  the  lateral,  concave  on  the  medial  sur- 
face, the  latter  being  called  the  hiliim  and  receiving  the  excretory 
duct  (ureter)  and  the  blood-vessels  of  the  organ  (hepatic  artery 
and  vein).  Just  inside  the  hilum  is  a  cavity,  the  pelvis  of  the 
kidney,  into  which  one  or  several  papillae  project,  each  bearing  the 


Fig.  330. — Lobulated  kidney 
(metanephros)  of  otter,  Lutra  cana- 
densis (Princeton,  2234).  a,  aorta; 
w,  ureter;  v,  postcava. 


UROGENITAL   SYSTEM.  33 1 

openings  of  numerous  collecting  tubules  (p.  309).  In  section  the 
substance  of  the  kidney  shows  two  different  textures,  recognizable 
to  the  naked  eye.  There  is  an  outer  cortical  and  an  inner  medul- 
lary substance,  the  two  interlocking  as  a  series  of  pyramids.  These 
different  appearances  are  due  to  the  fact  that  the  cortex  contains  the 
renal  corpuscles  and  convoluted  tubules,  while  the  medulla  is  com- 
posed of  the  straight  tubules  of  Henle's  loops  and  of  the  collecting 
system. 

The  ureters  are  free  for  most  of  their  course  from  the  kidney  to  the 
urinary  bladder,  into  which  they  enter  instead  of  the  cloaca.  The 
bladder,  in  the  monotremes  and  marsupials,  is  solely  allantoic  in 
nature,  but  in  the  placental  mammals  a  portion  of  the  cloaca  is  also 
included  in  it.  From  the  bladder  a  single  tube,  the  urethra,  leads  to 
the  exterior.  The  mammalian  urine  contains  urea  instead  of  uric 
acid,  a  resemblance  to  the  amphibia  and  a  contrast  to  the  sauropsida. 

REPRODUCTIVE  ORGANS  OF  THE  SEPARATE  GROUPS. 

CYCLOSTOMES. — The  gonads,  which  are  usually  unpaired,  are  supported  by 
a  fold  of  the  peritoneal  membrane  (mesorchium  or  mesovarium,  p.  122).  The  eggs 
and  sperm  escape  into  the  coelom  and  are  carried  thence  by  way  of  the  abdominal 
pores.  The  myxinoids  have  hermaphroditic  gonads,  the  anterior  part  being  female, 
the  posterior  testicular;  but  one  sex  predominates.  Nansen  believes  that  the  sexes 
alternate  in  function  (proterandric  hermaphroditism).  The  eggs  of  the  petromy- 
zonts  are  small,  those  of  the  myxinoids  are  larger  and  are  enclosed  in  a  horny  shell, 
with  anchoring  hooks  at  either  end. 

ELASMOBRANCHS. — In  the  elasmobranchs,  as  in  all  other  vertebrates,  the 
gonads  are  at  first  paired  and  symmetrical,  though  occasionally  one  side  or  the  other 
may  be  reduced  or  become  degenerate  or  those  of  the  two  sides  may  fuse.  Thus  in 
some  skates  only  the  left  gonad  may  be  functional.  Elsewhere  in  the  group  they  are 
paired  and  lie  far  forward,  attached  to  the  dorsal  wall  of  the  coelom.  The  Miillerian 
ducts  of  the  two  sides  in  the  female  meet  in  front  in  a  common  opening  (ostium 
tubae),  the  derivative  of  the  pronephric  nephrostomes.  This  receives  the  eggs, 
which  pass  from  the  ovaries  into  the  coelom.  The  diflferent  parts  of  the  duct  are 
specialized,  the  upper  part  serving  as  a  shell  gland,  forming  the  capsule  for  the 
eggs.  This  is  horny  and  in  most  species  is  provided  with  tendril  prolongations  at 
the  four  corners,  by  which  the  eggs  ('skate  barrows')  are  attached  to  submerged 
objects.  Some  species  of  both  sharks  and  skates  are  viviparous.  In  these  the 
lower  part  of  the  Miillerian  duct  (oviduct)  serves  as  a  kind  of  uterus.  In  some 
species  the  lining  of  this  uterus  is  covered  by  vascular  villi,  by  which  nourishment 
and  oxygen  are  conveyed  to  the  growing  young  which  escapes  in  approximately  the 
perfect  shape.  The  eggs  of  elasmobranchs  are  very  large,  those  of  some 
species  exceeding  even  those  of  the  ostrich  in  size. 


332 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


The  testes,  supported  by  mesorchia,  He  at  various  levels  in  the  coelom.  The 
relations  of  their  ducts  to  the  mesonephros  are  typical  (p.  521).  The  vasa  deferentia 
of  the  two  side  unite  just  before  entrance  into  the  cloaca  to  form  a  urogenital  sinus, 
with  which  an  oval  sperm  sac  is  connected  on  either  side.  In  Chimara  the  genital 
portion  of  the  mesonephros  (fig.  331)  is 
widely  separated  from  the  functional  por- 
tion, the  two  being  connected  by  the 
Wolffian  duct.  In  the  male  the  Miillerian 
duct  is  rudimentary  and  frequently  is  with- 
out a  lumen. 

GANOIDS.— Nothing  is  known  of  the 
development  of  the  sexual  organs  of  the 
ganoids,  except  as  to  the  origin  of  the  germ 
cells  in  two  species.  In  most  species  the 
ovary  is  band-like  and  the  oviducts  open  by 
broad  funnels  into  the  coelom,  but  in  Lepi- 
dosteus  the  ovary  is  saccular,  the  eggs  pass- 
ing into  the  central  cavity,  the  duct  being 
apparently  a  sterile,  backward  prolongation 
of  the  ovary.  In  the  male  the  testes  are 
frequently  lobulated  and  a  system  of  effer- 
ent ductules,  connected  by  a  longitudinal 
canal,  pass  from  the  testes  into  the  meso- 
nephros (fig.  325)  and  thence  separately 
or  by  a  single  tubule  into  the  Wolffian 
duct.  In  the  males  of  all  hut  Lepidosteus 
there  are  short  tubes  with  funnels,  appar- 
ently the  homologues  of  the  oviducts  of  the 
females. 

TELEOSTS. — In  some  of  the  lower 
teleosts  (salmonids,  etc.)  the  elongate  ovary 
is  solid  and  the  eggs  pass  from  it  into  the 
coelom  and  are  carried  thence  to  the  exterior 
by  short  peritoneal  funnels  (fig.  332),  or  the 
tubes  and  funnels  may  be  absent  and 
the  eggs  then  pass  out  by  abdominal  pores, 
is  a  closed  sac  (like  that  of  Lepidosteus,  fig.  326)  continued  behind  by  a  slender 
oviduct.  The  ducts  of  the  two  sides  may  open  separately,  but  usually  their  hinder 
ends  are  united  and  open  by  a  single  genital  pore  between  the  anus  and  the  rectum 
In  some  instances  (fig.  325,  E),  the  urinary  and  genital  pores  are  on  a  urogenital 
papilla.  In  the  male  the  elongate  testes  are  either  simple  or  lobulated.  Internally 
each  consists  of  radial  chambers  of  varying  shape  which  are  connected  with  a 
complicated  system  of  tubules  which  lead  to  a  vas  deferens  running  back  to  open 
into  the  hinder  end  of  the  Wolffian  duct,  or  separately  to  the  exterior  (fig.  333,  go). 

In  most  teleosts  the  number  of  eggs  produced  in  a  season  is  very  large,  sometimes 
numbering  millions.     Usually,  after  passing  from  the  oviducts,  they  are  left  to  the 


Fig.  331. — Testis  and  anterior  end 
of  mesonephros  of  Chimcera,  after  Par- 
ker and  Burland.  bv,  blood-vessel; 
cvl,  longitudinal  tubule;  m,  MuUerian 
duct;  ms,  anterior  end  of  mesonephros 
(Leydig's  gland);  spd,  sperm  duct;  ve, 
vet,  vasa  efferentia;  vs,  seminal  vesicle. 

In  most  teleosts,  however,  each  ovary 


UROGENITAL   SYSTEM. 


333 


mercy  of  the  water,  but  a  number  of  species  (Embiotocids,  Gambusia,  several 
Cyprinodonts,  etc.)  are  viviparous,  the  development  of  the  eggs  taking  place  in  the 
ovary,  which  sometimes  provides  nourishment  for  the  growing  young.  In  the 
lophobranchs  the  eggs  are  received  in  a  pouch  between  the  ventral  fins  of  the  male 
and  are  incubated  there.     Other  peculiar  breeding  habits  are  known. 


Fig.  332. — Relations  of  oviducts  and  peri  abdominales  in  Coregonus,  after  Weber. 
a,  anus;  i,  intestine;  n,  nephridial  opening;  o,  ovar>';  p,  pore  of  right  side;  r,  opening  of 
oviduct. 

DIPNOI.— In  the  dipnoi  more  normal  conditions  occur.  There  are  oviducts 
with  inner  ostia,  resembling  in  structure,  at  least,  the  Miillerian  ducts,  and  especially 
those  of  the  amphibia,  like  them  secreting  a  gelatinous  substance  around  the  eggs. 
These  same  ducts  are  also  retained  in  the  male  Ceratodus  and  to  a  less  extent  in 
the  other  genera  {Lcpidosiren  and  Protopterus).  The  gonads  are  long  and  are  cov- 


FiG.  7,T,T,. — Hinder  part  of  urogenital  organs  of  male  pike,  Esox  lucius,  after  Goodrich 
a,  anus;  ab,  air  bladder;  ao,  aorta;  d.  Wolffian  duct;  c,  cardinal  vein;  g,  genital  duct;  go, 
genital  opening;  i,  intestine;  pc,  postcardinalvein;M6,  urinary  bladder;  uo,  luinary  opening. 


ered  on  the  ventral  side  with  lymphoid  tissue.  The  testes  in  Protopterus  and 
Lepidosirm  contain  numerous  alveoli  lined  with  sperm-forming  cells.  The  sperm  is 
carried  into  a  longitudinal  tubule  (fig.  325)  and  from  thence  by  one  (Protopterus) 
or  several  efferent  ductules  to  the  Malpighian  bodies  of  the  posterior  end 
of  the  mesonephros,  the  epididymis  thus  being  posterior  in  position.  In  Ceratodus, 
which  is  imperfectly  known,  the  ductules  are  more  numerous  and  the  epididymis 
is  anterior. 

AMPHIBIA. — The  amphibians  are  the  most  typical  of  the  anamnia,  the  elasmo- 


334 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


branchs  excepted.  The  gonads  are  roughly  correlated  in  form  to  the  shape  of  the 
body,  being  shortest  in  the  anura,  longest  in  the  caecilians  and  urodeles.  The 
ovaries  are  saccular  (a  single  long  sac  in  urodeles,  a  number  of  short  ones  in  anura) 
and  the  eggs  pass  into  the  cavity  and  then  break  into  the  ccelom.  The  oviducts 
are  Miillerian  ducts  with  ostia  far  forward.  In  the  adults 
they  are  greatly  coiled  and  are  glandular,  their  walls  se- 
creting the  gelatinous  substance  which  envelops  the  eggs. 
Usually  the  oviducts  of  the  two  sides  open  separately  into 
the  cloaca,  but  the  two  unite  behind  in  Bufo. 

The  testes  have  both  the  longitudinal  and  the  testicular 
canals  connecting  the  efferent  ductules.  In  the  gymno- 
phiona  (fig.  334)  the  testes  resemble  a  string  of  beads,  each 
bead  consisting  of  a  number  of  seminiferous  sacs,  the 
string  being  united  by  the  testicular  canal.  The  efferent 
ducts  pass  through  the  mesonephros,  sometimes  utilizing 
the  nephridial  tubules,  sometimes  pursuing  a  separate  course, 
the  two  conditions  being  found  in  different  species  of  frog 
(Rana)  in  Europe.  Our  species  have  not  been  studied  in 
this  respect. 

The  cloaca  of  the  urodeles  has  a  glandular  lining  and  in 
the  females  it  contains  tubules  which  act  as  reservoirs  of 
sperm.  In  the  male  the  glands  secrete  a  substance  binding 
the  spermatozoa  together  In  many  urodeles  fertilization 
is  internal,  though  there  is  no  intermittent  organ  save  the 
somewhat  protrusible  cloacal  opening. 

There  are  many  interesting  accessory  reproductive  rela- 
tions among  the  amphibia.  Thus  the  cajcilians  and  Am- 
phiuma  lay  their  eggs  in  long  strings  in  the  soil  and  the 
female  incubates  them.  The  male  often  takes  charge  of  the 
eggs.  In  Pipa  each  egg  undergoes  development  in  a  pit  in 
the  skin  of  the  back  of  the  female  and  in  Nototrema  and 
Opisthodelphys  (South  America  tree-toads)  there  is  a  large 
pocket  in  the  skin  of  the  back,  opening  near  the  coccyx, 
where  the  eggs  are  carried  until  partially  {Nototrema)  or 
entirely  developed.  Salamandra  maculosa  and  S.  atra  bring 
forth  living  young,  the  former  being  born  with  gills,  the  latter 
in  the  perfect  condition.  Oviposition  usually  occurs  in  the 
spring  in  colder  climates  (in  the  autumn  with  Cryptohranchus 
of  America)  and  as  the  drain  on  the  system  is  very  consider- 
able immediately  after  hibernation,  the  substance  of  the 
fat  body,  which  always  is  closely  connected  with  the  gonads, 
is  utilized  at  this  time. 
SAUROPSIDA. — The  birds  and  reptiles  agree  in  the  broader  features  of  the 
amniote  urogenital  system  as  outlined  in  the  general  account  above.  There  is  a 
general  correlation  between  the  shape  of  the  body  and  that  of  the  gonads,  and  often 
there  is  a  lack  of  symmetry  between  the  organs  of  the  two  sides      Thus  in  snakes 


fcJ 


Fig.  334. — Male 
urogenital  organs  of 
Epicrium,  after 
Spengel.  a,  anus;  b, 
urinary  bladder;  cl, 
cloaca;/,  fat  bodies; 
m,  Miillerian  ducts; 
mg,  glandular  part 
of  same;  t,  testes;//, 
longitudinal  testi- 
cular canal;  w, 
Wolfl5an  body. 


UROGENITAL   SYSTEM. 


335 


and  lizards  the  gonad  of  one  side  is  in  advance  of  the  other,  while  in  forms  with 
large  eggs  there  is  a  marked  tendency  for  one  ovary  to  degenerate  (right  in  birds) 
the  other  alone  being  functional. 

The  oviducts,  which  are  Miillerian  ducts,  are  modified  in  accordance  with  the 
peculiarites  of  the  eggs.  The  upper  portion  is  usually  much  coiled  and  glandular, 
this  part  of  the  tube  secreting  the  white,  while  parts  farther  toward  the  external 
opening  form  the  shell  membrane  and  the  shell.  The  walls  are  also  somewhat 
muscular,   the  muscles  acting  like  constrictors  to  force  the  eggs  along.     The 


Fig.  335. — Model  of  cloacal  region  of  human  embryo,  6.5  mm.  long,  after  Keibel 
a,  allantois;  c,  cloaca;  cm,  cloacal  membrane;  k,  outgrowth  to  form  kidney  and  ureter; 
r,  rectum;  u,  where  bladder  will  develop;  wd,  Wolffian  duct. 


mesonephros  and  the  Wolfl5an  duct  are  largely  degenerate  in  the  female,  being 
represented  by  rudiments  between  the  oviduct  and  the  vertebral  column,  best 
developed  in  turtles  and  snakes. 

The  testes  (figs.  313,  328)  are  short,  round  or  oval  in  outline,  and  in  birds  one 
is  usually  the  larger,  though  both  increase  in  size  at  the  breeding  season.  The 
Wolffian  duct  is  solely  reproductive  (vas  deferens),  and  its  anterior,  greatly  coiled 
end,  together  with  the  vasa  ejGFerentia  form  the  epididymis.  Traces  of  the  Miillerian 
duct  persist  in  the  male  sauropsida.  There  are  several  accessory  reproductive 
glands  in  the  reptiles  but  little  is  known  of  their  function. 

MAMMALS. — In  considering  the  urogenital  structures  of  the  mam- 
mals the  following  parts  are  to  be  kept  in  mind :  They  are  composed  of 


336         COMPARATIVE  MORIHOLOGY  OF  VERTEBRATES. 

the  embryonic  excretory  organs  (mesonephroi)  and  their  (Wolffian) 
ducts;  the  permanent  kidneys  (metanephroi)  and  the  ureter;  the  gonads; 
the  Mullerian  ducts;  the  cloaca  and  the  anlagen  of  the  external  genitalia, 
which  arise  in  the  anterior  or  ventral  wall  of  the  urogenital  sinus. 

In  the  embryonic  stages  the  Wolffian  and  Mullerian  ducts  and  the 
ureters  open  into  the  cloaca  (fig.  335).  Then  a  part  of  the  latter,  with 
the  openings  of  these  ducts,  is  cut  off  to  form  the  allantois,  a  portion 
of  which  becomes  the  urinary  bladder,  this  part  receiving  the  ureters 


Fig.  336. — Model  of  pelvic  region  of  human  embryo  25  mm.  long,  after  Keibel.  (Com- 
pare with  fig.  335.)  a,  anal  opening;  /,  lateral  ligament  of  uterus;  w,  Mullerian  duct;  o, 
ovary;  pu,  primitive  ureter  (Wolffian  duct);  r,  rectum;  s,  symphysis  pubis;  sg,  septum  of 
genital  protuberance;  sug,  urogenital  sinus;  w,  ureter;  uh,  urinary  bladder;  ur,  recto-uterine 
excavation. 

(except  in  monotremes)  while  the  Wolffian  and  Mullerian  ducts  open 
into  the  basal  part  of  the  allantoic  outgrowth  which  is  separated  from 
the  bladder  by  a  narrower  stalk  which  becomes  the  urethra.  This 
part,  into  which  the  two  pairs  of  ducts  and  the  urethra  empty,  forms 
the  urogenital  sinus  (fig.  336,  sng).  With  the  formation  of  the  per- 
manent kidneys  the  mesonephros  largely  disappears  (see  p.  341)  and 
the  same  fate  extends  to  one  or  the  other  pair  of  ducts,  the  Mullerian 
largely  disappearing  in  the  male,  the  Wolffian  in  the  female.  The 
parts  which  persist  are  more  specialized  than  in  any  other  group  of 
vertebrates,  this  being  in  part  due  to  the  fact  that  usually  a  large  part 


UROGENITAL   SYSTEM. 


337 


of  the  development  of  the  young  is  passed  inside  the  body  of  the 
mother. 

In  their  early  stages  the  gonads  arise  anteriorly  to  the  permanent 
kidneys  and  they  retain  this  position  in  the  adult  monotremes  (fig. 
337).  In  all  others  they  are  gradually  carried  farther  posterior  in  the 
abdominal  cavity,  so  that  they  lie  on  the  caudal  side  of  the  kidneys. 


Fig.  337. — Urogenital  organs  of  male  Ornithorhynchus,  after  Gegenbaur.  6,  bladder; 
ep,  epididymis;  k,  kidney  opened,  showing  ends  of  collecting  tubules;  sr^  adrenal;  sug 
urogenital  sinus;  t,  testis;  ur,  ureter;  vd,  vas  deferens. 

This  transfer  of  position  is  effected  by  a  rather  complicated  apparatus, 
only  the  broader  features  of  which  can  be  outlined  here.  In  the  early 
stages  the  membranes  supporting  the  gonads  (mesorchia,  mesoaria) 
are  attached  to  the  medial  side  of  the  double  fold  of  the  serous  mem- 
brane around  the  mesonephros.  When  the  latter  organ  degenerates 
the  fold  becomes  the  broad  ligament  of  the  female,  while  another 


^^S  COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

fold  continues  down  the  genital  ducts  forming  the  ligament  of  the 
ovary  or  testis.  In  the  male  broad  ligament  and  ligamentum  testis 
together  form  the  gubernaculum.  Unequal  growth  of  body  and 
these  ligaments  draws  the  gonads  (except  in  the  monotremes)  farther 
back  into  the  pelvic  region. 

There  is  some  variation  in  the  ovaries.  In  the  monotremes  the  left  is  larger 
(cf.  birds)  and  it  is  interesting  to  note  that  eggs  have  been  found  only  in  the  left 
oviduct.  There  is  also  some  variation  in  shape  in  the  marsupials.  Elsewhere 
the  ovaries  are  relatively  small  (sometimes  increasing  in  size  at  the  breeding  season), 
rounded  or  oval  and  with  the  surface  smooth  or  furrowed. 

In  male  whales,  elephants,  some  edentates,  etc.,  the  testes  remain 
permanently  in  the  abdominal  cavity.  In  all  others  a  descent  of  the 
testes  occurs.  By  the  same  relative  difference  of  growth  of  body 
and  gubernaculum  the  testes  are  drawn  out  of  the  abdomen  into  a 
pouch  (scrotum) — ^really  a  part  of  the  body  wall  into  which  a  part 
of  the  coelom  (bursa  inguinalis)  extends.  The  wall  of  this  is  formed 
in  part  from  the  genital  folds  (see  copulatory  organs)  which  surround 
the  genital  prominence.  This  scrotum  is  in  front  of  the  penis  in  the 
marsupials,  behind  it  in  all  placentals.  When  the  canal  connecting 
the  cavity  of  the  bursa  with  the  rest  of  the  coelom  remains  open  (mar- 
supials, insectivores,  rodents,  bats,  etc.)  the  descent  is  temporary,  the 
testes  being  withdrawn  into  the  coelom  at  the  close  of  the  breeding 
season  by  a  ^cremaster  muscle.'  In  other  mammals  the  descent 
is  permanent,  though  in  some  species  it  does  not  occur  until  the  time 
of  sexual  maturity. 

In  the  oviducts  (Miillerian  ducts)  two  regions  can  be  recognized 
in  monotremes  (figs.  338,  339,  A),  three  in  all  other  forms.  The  two 
are  the  Fallopian  tube,  which  opens  into  the  body  cavity  by  a  broad, 
fringed  ostium  tubae,  and  second  the  uterus,  in  which  the  egg  is  retained 
for  a  part  of  its  development.  In  the  other  mammals  Fallopian  tube 
and  uterus  are  retained,  the  latter  being  specialized  for  the  longer 
development  of  the  young,  and  the  third  region  is  added — the  vagina, 
which  receives  the  copulatory  organ  of  the  male.  The  vagina  opens 
into  the  urogenital  sinus  (fig.  339,  B),  but  in  the  monotremes  the 
vagina  is  lacking  and  the  uterus  and  the  sinus  are  directly  connected. 
In  the  marsupials  a  vagina  is  developed  for  each  Miillerian  duct,  and 
in  some  there  is  a  peculiar  fusion  of  the  ducts  distal  to  the  vaginae  so 
that  a  caecal  pocket  results,  and  in  a  few  this  pocket  also  connects  with 
the  urogenital  sinus,  thus  forming  a  third  vagina  (fig.  339,  B). 


UROGENITAL   SYSTEM. 


339 


In  the  placental  mammals  the  posterior  (vaginal)  ends  of  the  two 
Miillerian  ducts  fuse  in  the  median  line,  thus  forming  a  single  vagina. 
In  some  the  two  uteri  remain  distinct,  each  having  its  own  opening 
(os  uteri)  into  the  vagina.  This  forms  the  uterus  duplex  (figs.  339,  J5, 
340,  //),  found  in  most  rodents.     In  carnivores,  ruminants,  horse  and 


'*-•-.;.-. ,_i--- 


FiG.  338. — ^Female  genitalia  of  Echidna,  after  Owen,  a,  openings  of  ureters  into, 
ug,  urogenital  sinus;  b,  bladder,  a  bristle  passing  into  urogenital  sinus;  c,  cloaca;  d,  opening 
of  rectum  into  cloaca;  o,  ovar>',  od,  oviduct,  the  lower  part  uterine,  r,  rectum;  w,  ureters. 


pig  the  fusion  has  been  carried  farther  so  that  there  is  a  single  os  uteri 
and  the  two  uteri  are  almost  completely  separated  (uterus  bipartitus, 
fig.  340,  ///)  or  the  fusion  is  carried  farther,  the  result  being  the' 
uterus  bicornis  (fig.  339,  C)  in  which  the  double  nature  is  still  shown 
by  the  two  pouches  at  the  upper  (anterior)  end.  Lastly,  in  the  pri- 
mates, the  fusion  of  the  two  primitive  uteri  is  complete,  the  result 
being  the  uterus  simplex  (figs.  339,  D;  340,  III-VI),  in  which  the 


340 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


double  nature  is  shown  only  by  the  separate  openings  of  the  two 
Fallopian  tubes. 

In  the  monotremes  the  primitive  relation  of  urogenital  sinus  and 
rectum — both  emptying  into  the  cloaca  (figs.  338,  339,  A) — persists 
through. life,  the  result  being  a  single  external  opening  for  the  digestive 


Fig.  339. — Uteri  of  {A)  Ornithorhynchus;  (B)  Halmaturus;  (C)  sheep  and  (D)  Inuus, 
after  Gegenbaur.  b,  bladder;  bo,  bursa  ovarica;  c,  cornua  uteri;  d,  cloaca;  /,  ligament  of 
ovary;  o,  ovary;  od,  oviduct  (Fallopian  tube);  pi',  processus  vaginalis;  sus,  sug,  urogenital 
sinus;  u,  uterus;  ur,  ureter;  v,  vagina;  vc,  vaginal  canals. 


tract  and  the  urogenital  ducts,  whence  the  name  monotreme.  In 
all  other  mammals  the  cloaca  becomes  divided  by  a  partition,  the 
perinaeum,  between  the  urogenital  and  the  rectal  portions,  there  thus 
being  formed  two  external  openings.  However,  in  certain  mammals, 
as  in  marsupials  and  some  rodents,  both  may  be  enclosed  in  a  common 


UROGENITAL   SYSTEM. 


341 


fold  of  integument  (fig.  341)  and  in  the  former  group  may  be  provided 
with  a  common  sphincter  muscle. 

The  testes  are  relatively  small  and  the  outer  surface  is  smooth  as 
the  result  of  the  development  around  them  of  a  fibrous  envelope,  the 
tunica  albuginea.  This  sends  inward  partitions  (trabeculae)  which 
separate   groups   of   seminiferous   tubules   into   lobules.    From   the 


Fig.  340. — Modifications  of  female  urogenital  structures  in  /,  monotreme;  //, 
Orycteropus  (uterus  duplex);  ///,  many  monodelphs  (uterus  bipartitus); /F,  most  mono- 
delphs;  V,  Bradypus;  VI,  Dasypus;  b,  bladder;  c,  urinary  canal,  cu,  urogenital  sinus;  g, 
genital  sinus;  o,  oviduct,  u,  uterus;  v,  vagina. 


lobules  the  sperm  is  carried  outward  by  numbers  of  small  tubules,  the 
homologues  of  the  efferent  ductules  of  the  lower  vertebrates,  and 
like  them  connected  together  by  vessels  which  correspond  to  the  longi- 
tudinal canals.  The  ductules  empty  into  the  anterior  end  of  the 
Wolffian  duct,  the  upper  end  of  which  is  greatly  coiled,  the  coiled  por- 
tion and  the  ductules  forming  the  epididymis.  From  the  entrance 
of  the  ductules  to  its  entrance  into  the  urogenital  sinus  or  canal  the 
duct  is  called  the  vas  deferens.  From  this  point  the  urogenital  canal 
is  provided  with  muscular  walls  and  forms  an  ejaculatory  duct. 

In  the  female  the  Wolffian  duct  and  the  mesonephros  are  largely  lost  in  the 
adult,  the  mesonephros  forming  a  small  collection  of  tubules  near  the  anterior 
end  of  the  ovary  which  are  known  as  the  parovarium.     In  the  male  the  Miillerian 


342        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

duct  is  also  largely  lost,  the  lower  portion  sometimes  persisting  as  a  small  blind 
tubule  imbedded  in  the  prostate  gland  and  known  as  the  uterus  masculinus. 

In  the  testes  between  the  tubules  are  small  aggregates  of  cells  known  as  inter- 
stitial cells,  which  have  recently  been  shown  to  be  glands  with  internal  secretion. 
In  man  their  products,  which  pass  into  the  blood,  apparently  cause  the  assumption 
of  the  secondary  male  characters — growth  of  hair  on  the  face,  change  of  voice,  etc. 
— at  the  time  of  puberty.  There  would  also  seem  to  be  some  analogous  structure 
in  the  ovary  governing  the  development  of  female  characteristics  and  controlling 
some  of  the  features  of  menstruation. 

There  are  a  number  of  accessory  glands  connected  with  the  genital  ducts,  these 
being  usually  better  developed  in  the  male  than  in  the  female.     Only  the  more 


Fig.  341 . — Diagram  of  male  genitalia  of  beaver,  Castor  canadensis,  after  Weber,  a, 
anus;  ag,  anal  gland;  b,  urinary  bladder;  gv,  gland  of  vas  deferens;  oa,  opening  of  anal  gland; 
op,  OS  penis;  p,  prostate;  pp,  preputial  gland;  r,  rectum;  u,  ureter;  vd,  vas  deferens. 

prominent  are  mentioned  here.  The  seminal  vesicles  (present  in  some  rodents, 
bats,  insectivores  and  in  ungulates  and  primates)  are  a  pair  of  tubular  or  saccular 
glands  opening  into  the  vasa  deferentia  just  before  their  entrance  into  the  urogenital 
canal.  The  prostate  glands,  which  occur  in  all  placental  mammals  with  the 
exceptions  of  edentates  and  whales,  are  connected  with  the  urogenital  canal. 
Farther  along  the  canal  are  Cowper's  glands  which  occur  in  almost  all  mammals 
as  scattered  bodies  or  aggregated  into  larger  masses,  and  surrounded  by  smooth 
muscle. 

Concerning  the  functions  of  these  glands  considerable  uncertainty  exists. 
From  the  fact  that  removal  of  the  prostate  and  the  seminal  vesicle  in  rats  prevented 
fertilization,  and  the  further  fact  that  the  secretion  of  the  seminal  vesicles  increases 
the  activity  of  the  spermatozoa,  it  seems  probable  that  they  are  of  great  importance 
in  connexion  with  fertilization.  Then  it  has  been  shown  that  in  some  instances 
the  coagulation  of  the  secretion  of  these  glands  closes  the  vagina  after  copulation 
has  occurred,  thus  preventing  the  exit  of  the  sperm. 

COPULATORY  ORGANS. 

In  many  vertebrates  the  eggs  are  fertilized  after  passing  from  the 
oviducts.  This  is  the  case  with  the  cyclostomes,  most  fishes,  with 
the  exception  of  the  elasmobranchs,  and  with  many  amphibians.     In 


UROGENITAL  SYSTEM. 


343 


Other  groups  fertilization  is  internal.  In  some  cases  the  transfer  of 
the  sperm  from  the  male  to  the  female  is  effected  by  the  apposition 
of  the  cloacae  of  the  two  sexes,  but  in  others  copulatory  organs  of  an 
intromittent  character  occur.  These  are  formed  on  several  plans  and 
are  not-  homologous  throughout. 


Fig,  342. — Hemipenes  of  Crotalus  horridus,  after  J.  Miiller.  One  hemipenis  is 
exserted,  the  other  retracted  but  laid  open,  cl,  cloaca;  g,  seminal  groove;  p,  hemipenis; 
r,  rectum,  rp,  retractor  muscle  of  hemipenis;  u,  ureter;  vd,  vas  deferens  (Wolffian  duct)."*^ 

In  the  male  elasmobranchs  the  posterior  or  inner  side  of  the 
pelvic  fins  are  specialized  for  this  purpose.  The  metapterygium 
(p.  116)  and  the  basalia  connected  with  it  are  more  or  less  completely 
separated  from  the  rest  and  form  the  so-called  clasper  (*mixip- 
terygium').     Each  of  these  is  grooved  along  its  medial  surface  and 


344 


COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 


when  the  two  are  inserted  in  the  cloaca  the  grooves  unite  to  form  a 
tube  for  the  passage  of  the  sperm.  There  is  a  large  gland  in  the 
clasper  but  its  relation  to  copulation  and  fertilization  is  unknown. 

In  the  snakes  and  lizards  a  second  kind  of  structures  occurs.  In 
the  young  there  are  developed  behind  the  vent  a  pair  of  sacs  presenting 
the  appearance  of  appendages.  With  farther  growth  these  two 
hemipenes  are  withdrawn  into  a  sac  opening  into  the  hinder  side  of 


Fig.  343. — Cloacal  region  of  adult  turtle  {Emys  lutaria),  after  von  Moller.  The 
rectum  and  cloaca  have  been  laid  open  from  the  dorsal  surface  and  the  urogenital  sinus 
exposed.  From  the  opening  of  the  sinus  into  the  cloaca  a  seminal  groove  extends  along 
the  ventral  cloacal  surface  and  can  be  cut  off  by  a  pair  of  folds  (plicce  urorectales)  from  the 
cloacal  cavity,  av,  anal  vesicle;  h,  urinary  bladder;  o,  opening  of  anal  vesicle  into  cloaca; 
p,  penis,  exserted;  pu,  plicae  urorectales;  r,  rectum;  sg,  seminal  groove;  ug,  urogenital 
groove. 

the  cloaca.  Each  hemipenis  bears  a  spiral  groove  for  the  passage  of 
the  sperm.  At  the  time  of  copulation  these  are  everted  through  the 
anus  (fig.  342). 

In  all  other  aminotes  the  copulatory  organs  are  formed  from  the 
same  anlage.  The  lower  anterior  wall  of  the  cloaca  is  largely  con- 
cerned in  this,  the  anterior  cloacal  lip  being  produced  into  a  genital 
prominence  (fig.  336)  which  can  be  traced  in  many  forms  as  the 
clitoris  of  the  female  and  the  glans  penis  of  the  male.  In  the  embryos 
of  the  higher  mammals  it  is  surrounded  by  a  pair  of  integumental 


UROGENITAL   SYSTEM. 


345 


folds  which  develop  into  the  labia  of  the  genital  opening  in  the  female 
while  in  the  male  they  furnish  a  part  of  the  scrotal  envelope. 

The  most  primitive  type  of  the  cloacal  penis  is  found  in  the  chel- 
onians  (fig.  343)  and  crocodiles,  and  slightly  more  developed  in  the 


Fig.  344. — Ventral  cloacal  wall  and  penis  of  Rhea  (schematized),  after  Boas,  b, 
blind  sac;/,  corpus  fibrosum;  g,  seminal  groove;  g',  its  continuation  along  blind  sac;o, 
opening  of  blind  sac.     Mucous  membrane  dotted,  seminal  groove  black. 

ostriches  and  some  of  the  aquatic  birds.  In  these  the  ventral  or 
anterior  wall  of  the  cloaca  and  its  lip  become  specialized  by  the  develop- 
ment in  it  of  a  longitudinal  band  of  fibrous  tissue,  covered  on  the 
cloacal  side  by  cavernous  tissue  (containing  large  spaces,  which  on 


Fig.  345. — Diagrams  of  male  urogenitalia  in  /,  monotreme;  II,  marsupials;  and  III; 
monodelphs,  after  Weber,  a,  anus;  b,  bladder;  c,  cloaca;  cc,  corpus  cavern osus  urethra, 
cp,  Corp.  cav.  penis;  cd,  Cowper's  gland;  p,  perinasum;  pg,  prostate  gland;  r,  rectum;  s, 
symphysis  pubis;  /,  testis;  u,  ureter;  v,  vas  deferens;  vg,  vesicular  gland;  i/m,  ventral  muscles. 


being  filled  with  blood  render  the  whole  firm  and  enlarged — erectile 
tissue).  The  cavernous  tissue  is  marked  by  a  longitudinal  groove 
through  which  the  seminal  fluid  from  the  urogenital  sinus  runs.  Be- 
sides the  enlargement  caused  by  the  filling  of  the  cavernous  tissue  with 


346        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

blood,  the  whole  structure,  the  distal  end  of  which  is  free,  can  be 
protruded  from  the  cloaca  and  retracted  by  suitable  muscles  (fig.  344) . 
In  the  monotremes  (fig.  345,  /)  the  penis  is  still  cloacal  in  position 
and  the  urogenital  sinus  still  communicates  with  the  cloacal  cavity. 
But  the  advance"  is  made  that  the  groove  of  the  sauropsida  has  been 
converted  into  a  tube  which  carries  the  urine  as  well  as  the  sperm. 
The  whole  structure  can  be  protruded  and  retracted  again  into  a 
sheath  formed  from  the  loose  mucous  membrane  of  the  cloaca.  In 
the  other  mammals  the  connection  of  the  urogenital  ducts  with  the 
alimentary  tract  is  lost  and  the  cloaca  disappears.  In  the  lower 
mammals  (figs.  341,  345,  //)  the  retractile  condition  is  retained  but 
in  the  higher  the  organ  is  permanently  external  (fig.  345,  ///).  In 
the  marsupials  the  tip  of  the  penis  is  frequently  bifurcate,  corresponding 
to  the  two  vaginae  of  the  female.  In  many  rodents  (fig.  341,  op)j 
bats,  many  carnivores,  whales  and  some  of  the  primates  a  penis  bone 
is  developed  in  the  middle  line  of  the  intromittent  organ. 

HERMAPHRODITISM. 

Individuals  of  either  sex  which  have  assumed  some  of  the  external 
or  secondary  sexual  characters  of  the  other  sex  are  sometimes  spoken 
of  as  hermaphrodites,  especially  in  the  case  of  mammals  if  the  copu- 
latory  organs  be  concerned.  This  is  not  true  hermaphroditism,  which 
consists  in  having  both  ovarian  and  testicular  organs  or  tissues  in  the 
same  individual  and  as  a  consequence  the  ability  to  produce  both  eggs 
and  spermatozoa.  There  may  be  both  kinds  of  tissue  in  the  different 
parts  of  the  same  gonad,  or  the  two  may  be  intermingled  (ovotestis) 
or  the  gonads  of  the  two  sides  of  the  body  may  be  of  different  sexes. 
Both  ovaries  and  testes  may  be  functional  at  the  same  time,  or  one 
may  be  functional  at  one  time  and  the  other  at  another  (proterandric 
hermaphroditism) . 

There  is  an  enormous  literature  dealing  with  the  problem  of  the 
determination  of  sex.  Almost  every  conceivable  possibility  has  been 
invoked  to  account  for  fact  that  one  individual  is  male  and  another 
female — chance,  multiple  impregnation,  difference  in  age  of  parents 
or  of  eggs  and  spermatozoon,  matters  of  temperature  and  nutrition, 
etc.  Within  the  last  few  years  there  has  been  a  strong  tendency  to 
regard  the  matter  as  determined  at  the  time  of  impregnation  of  the 
egg  and  to  depend  upon  differences  in  chromosomes. 


UROGENITAL   SYSTEM.  347 

In  the  formation  and  maturation  of  spermatozoa  and  eggs  a  peculiar  substance  in 
the  nucleus— chromatin— becomes  aggregated  in  small  bodies  called  chromosomes, 
the  number  of  which  in  the  mature  genital  products  is  half  of  that  occurring  in 
the  other  cells  of  the  body.  In  most  species  the  number  in  the  body  cells  is  always 
even  and  is  therefore  exactly  divisible,  but  it  was  found  that  in  certain  insects  there 
were  differences  between  the  sexes,  the  male  having  an  odd,  the  female  an  even 
number.  When  the  reduction  division  occurs,  by  which  the  chromosomes  are 
divided  between  the  mature  eggs  or  the  spermatozoa  (for  details  see  cytological 
works),  the  eggs  would  all  have  the  same  number  of  chromosomes  while  the 
spermatozoa  would  be  dimorphic,  some  having  an  odd  and  some  an  even 
number  of  chromosomes.  In  other  cases  there  is  frequently  one  or  more 
chromosomes  (idiochromosomes)  which  differ  from  the  rest,  and  these  are  dis- 
tributed in  the  same  way  at  the  reduction  division.  At  the  fertilization  of  the 
egg  there  is  an  addition  of  the  chromosomes  of  the  spermatozoa  to  those  of  the 
egg,  consequently  some  of  the  eggs  will  have  the  odd  number  and  some  the 
even  number  of  chromosomes,  this  being  perpetuated  in  all  of  the  cells  of  the 
resulting  organism  until  the  next  reduction  division.  It  would  thus  follow  that 
sex  was  determined  at  the  time  of  fertilization  of  the  egg.  But  this  is  difficult 
to  reconcile  with  the  existence  of  hermaphroditism. 

Another  view,  which  better  accords  with  the  facts,  is  that  sex  is  a  matter  of 
Mendelian  inheritance,  the  females  in  some  instances  being  heterozygous,  the 
males  homozygous;  or  these  relations  may  be  reversed  In  the  first  condition 
the  element  of  'femaleness'  dominates  over  the  recessive  'maleness'.  In  such 
cases  it  seems  reasonable  to  suppose  that  the  hermaphrodites  are  really 
heterozygous  females  in  which  the  normally  recessive  'maleness'  has  become 
equally  potent  with  the  female,  while  under  ordinary  conditions  the  matter  of 
sex  is  dependent  upon  the  character  of  the  chromosomes  combined  with  the 
Mendelian  inheritance. 

Among  the  cyclostomes  there  are  occasional  specimens  of  lam- 
preys which  have  been  regarded  as  hermaphroditic,  but  in  the  myx- 
inoids  this  is  the  regular  occurrence,  the  anterior  end  of  the  gonad 
is  male  and  the  posterior  female.  One  or  the  other  of  these  is  func- 
tional, the  animal  being  predominantly  either  male  or  female,  and 
some  individuals  are  regarded  as  sterile.  Nansen  regards  this  as  a 
case  of  proterandric  hermaphroditism.  In  the  teleosts  several  species 
of  Serranus  are  regularly  hermaphroditic  as  is  Chrysophrys  aurata, 
while  in  several  other  species  it  is  an  occasional  occurrence.  Triton 
tcBfiiatus  is  the  only  urodele  in  which  it  is  reported,  but  in  the  anura  it 
is  more  common.  Thus  it  is  frequent  in  the  frogs  and  occasional  in 
other  genera.  In  the  toads  (Bufo)  there  is  frequently  a  *  Bidder's 
organ'  in  front  of  the  gonads  which  contains  immature  ova  in  the 
male.  Among  the  birds  the  phenomenon  has  been  reported  in  the 
chaflSnch.     (The  assumption  of  male  plumage  by  female  birds  at  the 


348  COMPARATIVE   MOGPHOLOGY    OF   VERTEBRATES. 

close  of  sexual  life  is  not  a  case  of  hermaphroditism.)  Among  the 
mammals  the  cases  are  extremely  rare,  but  cases,  apparently  well 
authenticated,  have  been  reported  in  the  goat,  pig  and  man. 

NUTRITION  AND  RESPIRATION  OF  THE  EMBRYO— FOETAL 

ENVELOPES. 

In  all  vertebrates  except  the  mammals  there  is  enough  nourish- 
ment stored  in  the  egg  to  carry  the  young  through  its  development 
up  to  the  point  where  it  hatches  and  shifts  for  itself.  In  the  cyclo 
stomes,  dipnoi  and  amphibia  this  nourishment  (food-yolk  or  deuto- 
plasm)  is  soon  enclosed  in  the  body  wall.  In  ganoids  and  teleosts, 
where  it  is  relatively  larger  in  amount,  it  forms  for  a  time  a  projecting 
mass  enclosed  in  a  yolk  sac,  and  this  condition  reaches  its  extreme  in 
the  elasmobranchs  and  sauropsida.  The  yolk  sac,  in  the  fishes,  is  an 
extension  of  the  intestine  and  the  body  wall  and  is  richly  supplied  by 
vitelline  arteries  and  veins  which  are  derivatives  of  the  omphalo- 
mesenteric vessels  (p.  276).  In  the  sauropsida,  owing  to  the  develop- 
ment of  the  amnion  and  the  consequent  separation  of  the  non- 
embryonic  somatopleure  from  the  yolk,  the  yolk  sac  is  composed  of 
the  splanchnopleure  alone,  but  it  has  homologous  blood-vessels.  In 
the  mammals  (monotremes  excepted)  the  yolk  is  greatly  reduced  and 
the  yolk  sac  (here  often  called  the  umbilical  vesicle)  is  vestigial  in 
character. 

The  vitelline  vessels  take  the  yolk  and  carry  it  into  the  body  where 
it  is  utilized  in  building  the  embryo,  all  of  it  being  eventually  metabo- 
lized and  used  by  the  cells.  The  rich  supply  of  capillary  vessels  in  the 
sac  also  forms  an  efficient  respiratory  apparatus.  In  the  viviparous 
sharks  villi  are  developed  on  the  oviducal  lining  and  these  afford  a 
means  of  exchange  of  gases  with  the  embryo,  and  for  getting  rid  of  the 
nitrogenous  waste.  It  is  a  question  how  far  there  is  a  transfer  of  food 
by  the  same  means.  In  some  species  of  Mustelus  and  Carcharias 
the  villi  fit  into  depressions  in  the  yolk  sac,  thus  forming  an  analogue 
to  the  placenta  of  the  mammals —  a  vitelline  placenta — though  formed 
in  a  greatly  different  manner. 

The  viviparous  teleosts  have  saccular  ovaries  and  the  development 
of  the  egg  takes  place  in  the  cavity,  the  walls  of  which  at  the  breeding 
season  become  villous.  In  the  viviparous  Salamandra  atra  only  one 
egg  develops  and  this  leaves  the  mother  in  the  adult  shape.  The 
other  eggs  degenerate  and  are  used  as  food  by  the  one.     There  is  also 


UROGENITAL    SYSTEM.  349 

a  modification  of  the  lining  of  the  oviduct  in  this  species  which  allows 
some  blood  to  escape  and  this  gives  additional  nourishment. 

In  the  amniotes  the  yolk  sac  reappears  and  there  are  in  addition 


Fig.  346. — Diagrams  of  the  development  of  amnion  and  allantois.  Upper  figure  earlier, 
transverse  section;  lower  later,  longitudinal,  a,  amnion;  al,  alimentarj'  canal;  am.  cav, 
amniotic  cavity;  ch,  beginning  of  chorion;  5,  serosa;  so,  somatopleure ;  ys,  yolk  stalk. 


350        COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

two  other  embryonic  structures  which  are  peculiarly  characteristic, 
the  aUantois  and  the  amnion,  to  which  reference  has  been  made  before. 
The  amnion  arises  as  a  fold  of  the  somatic  wall  of  the  coelom  in 
front  of  and  on.  either  side  of  the  embryo.  These  folds  extend  upward 
and  then  inward  until  they  finally  meet  above  the  embryo,  thus  en- 
closing it  in  an  amniotic  cavity.  The  folds  fuse  in  the  middle  line  and 
then  the  two  sides  break  through  so  that  above  the  wall  of  the  amniotic 
cavity — the  true  amnion — there  is  a  second  cavity  directly  continuous 
with  the  coelom,  and  this  is  bounded  externally  by  the  rest  of  the 
amniotic  fold,  this  part  being  called  the  serosa  or  false  amnion.  This 
lies  immediately  beneath  the  vitelline  membrane  of  the  egg  or  its 
equivalent,  to  which  many  different  names  have  been  given. 

Little  is  known  as  to  the  phylogeny  of  the  amnion,  a  structure  without  parallel 
in  the  animal  kingdom  except  in  the  scorpions,  where  one  is  formed  in  the  same 
way.  Of  course  there  is  no  genetic  connexion  between  the  two.  It  has  been  sug- 
gested that  in  both  groups  there  is  a  tendency  for  the  embryo  to  sink  into  the  yolk 
and  that  the  amnion  is  to  prevent  its  being  completely  covered  with  this  substance. 

The  homologue  of  the  aUantois  is  found  in  the  urinary  bladder  of 
the  amphibia.  It  is  an  outgrowth  from  the  hinder  end  of  the  alimentary 
tract  and  consists  of  a  lining  of  entoderm,  covered  externally  with  the 
splanchnic  layer  of  the  mesoderm — is  purely  splanchnopleuric — and 
projects  into  the  coelom.  In  its  outgrowth  it  carries  with  it  branches 
of  the  hypogastric  blood-vessels,  now  known  as  the  allantoic  arteries 
and  veins  (usually  but  a  single  vein).  As  it  develops,  the  distal 
end  of  the  aUantois  swells  into  a  large  vesicle,  connected  with  the 
digestive  tract  by  a  slender  stalk.  The  vesicle  extends  into  the  coelom 
between  the  amnion  and  serosa  and  soon  fuses  with  the  serosa.  The 
terminal  sac  flattens  and  gradually  extends  until  it  encloses  the  whole 
embryo  and  amniotic  sac. 

In  the  sauropsida  the  aUantois  (and  serosa)  comes  eventually  to 
lie  just  beneath  the  shell,  and  as  the  latter  is  porous  and  the  aUantois 
is  very  vascular,  the  latter  is  in  position  to  act  as  the  respiratory 
apparatus  of  the  growing  young.  The  cavity  of  the  aUantois,  con- 
nected by  its  stalk  with  the  cloacal  region,  serves  as  the  reservoir  for 
the  urine. 

While  the  embryo  is  increasing  in  other  respects,  the  side  walls  of 
the  body  gradually  close  in  ventral  to  the  embryo  until  they  reach  the 
stalks  of  the  yolk  sac  and  the  aUantois.  In  this  way  these  structures 
come  to  be  connected  with  the  body  by  a  narrow  cord,  called  in  mam- 


UROGENITAL   SYSTEM.  35 1 

mals  the  umbilical  ornavel  cord,  in  which  the  blood-vessels  run.  In 
the  mammals  there  are  several  variations  from  the  above  account  of  the 
development  of  the  allantois,  but  they  can  be  reconciled  with  the 
typical  condition  in  the  sauropsida.  There  are  also  several  other 
variations  and  the  relations  of  allantois  to  the  other  structures  is  more 
complicated,  but  details  and  the  many  modifications  must  be  ignored 
here,  only  an  outline  of  the  broader  features  being  given. 

In  the  mammals  there  is  the  same  fusion  of  allantois  and  serosa  as 
in  the  sauropsida,  the  fused  area  here  being  called  the  chorion.  On 
arrival  in  the  uterus  by  way  of  the  Fallopian  tube,  the  egg  becomes 
implanted  in  the  uterine  wall,  and  a  little  later,  with  the  development 
of  the  chorion,  villi  are  formed  on  the  outer  surface  of  the  egg.  These 
are  invaded  by  the  chorionic  blood-vessels  and  they  branch  and  extend 
into  depressions  or  crypts  in  the  walls  of  the  uterus.  The  latter  become 
very  vascular,  the  blood  spaces  of  the  maternal  tissue  enveloping  the 
villi  with  only  the  thinnest  of  walls  between  the  vessels  of  the  mother 
and  those  of  the  young.  (There  is  never  any  actual  connexion  between 
the  blood-vessel  of  parent  and  embryo  and  so  blood  corpuscles  cannot 
pass  from  one  to  the  other.  All  that  takes  place  is  largely  of  the  nature 
of  osmosis — solutions  of  gases,  of  nourishing  substances  and  of  nitrog- 
enous waste  passing  from  one  to  the  other.  There  is  difiSculty  in 
explaining  the  passage  of  proteids  and  fats.)  This  structure,  consisting 
of  the  allantoic  derivatives  of  the  embryo  and  the  mucous  lining  of  the 
uterus,  is  known  as  the  placenta. 

In  the  monotremes  and  in  most  marsupials  no  placenta  is  formed, 
but  it  has  been  recently  shown  that  a  true  placenta  occurs  in  a  few  of  the 
latter  group.  In  other  mammals  a  placenta  always  occurs,  the  struc- 
tures presenting  many  forms,  but  these  may  be  grouped  under  a  few 
heads.  (It  must  be  borne  in  mind  that  this  classification  is  purely 
morphological  and  does  not  necessarily  imply  close  relations  of  the 
species  included  or  identity  of  method  of  formation.) 

In  many  mammals,  at  the  time  of  birth,  the  maternal  and  embryonic 
parts  of  the  placenta  simply  separate,  only  the  latter  passing  away 
with  the  young.  These  are  called  non-deciduate  placentae.  In 
the  others  the  union  of  the  foetal  and  the  maternal  tissues  is  so  intimate 
that  the  inner  surface  of  the  uterus  is  included  in  the  afterbirth.  These 
form  the  deciduate  type.  The  non-deciduata  include  two  divisions. 
In  the  diffuse  placentae  (edentates,  whales,  perissodactyls,  many 
artiodactyls)  the  villi  are  distributed  over  the  entire  surface  of  the 


352         COMPARATIVE  MORPHOLOGY  OF  VERTEBRATES. 

chorion.  In  the  cotyledonary  placenta  the  villi  are  grouped  in  small 
areas  (cotyledons)  with  spaces  of  naked  chorion  between  them.  This 
form  is  characteristic  of  the  ruminants.  The  deciduate  type  includes 
the  zonary  and  the  discoidal  forms.  In  the  zonary  placenta  (eden- 
tates, sirenians,  elephants,  hyracoids  and  carnivores)  the  villi  form  a 
girdle  around  the  placental  sac,  the  ends  of  the  chorion  being  free  from 
them.  In  the  discoidal  forms  (insectivores,  rodents,  bats,  edentates, 
primates)  the  villi  are  restricted  to  one  side  of  the  chorion. 

ADRENAL  ORGANS. 

Under  this  heading  are  included  two  sets  of  structures,  interrenals 
and  suprarenals,  of  uncertain  morphology  and  function.  The  names 
are  given  in  allusion  to  the  fact  that  they  are  usually  closely  associated 
in  position  with  the  nephridial  structures,  though  they  have  no  other 
relation  to  them.  The  two  differ  in  structure  and  probably  in  function 
and  are  very  distinct  in  the  lower  vertebrates  but  in  amphibia  and 
amniotes  they  are  united  in  a  common  structure,  the  interrenals  forming 
the  cortex,  the  suprarenals  the  medulla  of  the  mammalian  adrenals. 

The  interrenals  arise  from  the  coelomic  epithelium  but  it  is  as  yet 
uncertain  as  to  the  details,  some  thinking  that  they  are  connected  with 
the  pronephros,  others  with  the  mesonephric  structures,  while  still  others 
regard  them  as  distinct  in  origin.  They  are  at  first  either  isolated 
clusters  of  cells  or  longer  bands  of  cells  near  the  dorsal  margin  of  the 
mesentery,  sometimes  bilaterally  symmetrical  and  in  the  lower  verte- 
brates extending  through  the  length  of  the  cpelom. 

The  suprarenals  find  their  anlage  in  the  sympathetic  ganglia,  from 
which  certain  cells  early  separate.  Among  these  are  peculiar  cells 
which  are  called  chromafifin  cells  (chromaphile  or  phaeochrome 
cells)  because  of  their  staining  brown  or  yellow  with  chromic  acid 
salts.  These  usually  are  closely  associated  with  the  blood-vessels, 
either  the  dorsal  branches  of  the  segmental  arteries  or  the  postcardinal 
veins. 

In  the  fishes  the  two  organs  are  separate,  the  suprarenals  often 
being  more  or  less  metameric  in  character,  and  in  close  relations  to 
the  vessels  of  the  mesonephros.  The  interrenals  form  more  compact 
organs  between  the  nephridia  of  the  two  sides.  In  all  tetrapoda  the 
two  organs  are  more  closely  associated,  the  tissues  of  the  two  being 
mixed  in  the  adults  of  the  amphibia  and  reptiles,  while  in  the  mammals 


UROGENITAL    SYSTEM.  353 

the  interrenal  tissue  is  on  the  outer  side  of  the  adrenal  organ,  the  su- 
prarenal forming  the  inner  portion.  In  the  amphibia  the  adrenals  are 
closely  connected  with  the  mesonephroi,  being  attached  to  their 
inner  margins  (urodeles)  or  to  the  ventral  surface  (anura).  In  the 
reptiles  they  are  lobulated  structures  near  the  gonads.  In  the  mammals 
they  are  more  compact  (often  called  suprarenals)  and  are  placed  at  the 
anterior  end  of  the  kidneys,  often  unsymmetrically. 

Both  organs  are  regarded  as  glands  of  internal  secretion,  their 
product  being  passed  directly  into  the  blood.  The  secretion  of  the 
medullary  portion  (suprarenal)  of  the  mammals  is  adrenalin,  an  acti- 
vator or  hormone,  which  by  its  action  on  the  muscular  system  causes 
an  increase  in  the  blood  pressure.  Even  less  is  known  of  the  function 
of  the  interrenal.  Certain  observ^ations  render  it  probable  that  the 
secretion  of  this  is  of  value  in  destroying  certain  products  of  metabolism 
which  otherwise  might  be  injurious  to  the  organism. 


2Z 


BIBLIOGRAPHY. 

In  this  list  of  books  and  articles  dealing  with  vertebrate  morphology  there  have  been  in- 
cluded only  such  titles  as  are  likely  to  be  accessible  in  the  majority  of  the  laboratories  of 
the  countr}'.  Hence  citations  are  largely  from  the  periodicals  and  society  publications  of 
America  and  England  and  from  the  leading  journals  of  the  Continent.  The  student  who 
wishes  to  go  farther  into  any  subject  will  find  additional  references  in  the  papers  quoted 
here  and  also  in  the  works  of  Wiedersheim,  Gegenbaur,  Hertwig  and  others,  while  the  cur- 
rent papers  are  listed  in  the  Anatomischer  and  Zoologischer  Anzeigers.  For  economy  of 
space  the  titles  have  been  abbreviated,  but  in  such  a  way  as  to  indicate  something  of  the 
character  and  contents  of  the  work. 

JOURNALS  Ain)  TRANSACTIONS. 

Academy  of  Natural  Sciences,  Philadelphia,  Proceedings. 

American  Naturalist. 

American  Journal  of  -Anatomy. 

American  Academy  of  Arts  and  Sciences,  Proceedings. 

Anatomical  Record. 

Anatomischer  Anzeiger. 

Anatomische  Hefte. 

Archiv  fiir  Anatomie  und  Physiologic,  Anatomische  Abtheilung. 

Archiv  fiir  mikroscopische  Anatomie. 

Biological  Bulletin. 

Boston  Society  of  Natural  Histor}',  Memoirs  and  Proceedings. 

Ergebnisse  der  Anatomie  und  Entwicklxmgsgeschichte. 

Jenaische  Zeitschrift  fiir  Naturwissenschaften. 

Journal  of  Anatomy  and  Physiology. 

Journal  of  Comparative  Neurology. 

Journal  of  Morphologv'. 

Mittheilungen  aus  der  zoologischen  Station  zu  Neapel. 

Morphologische  Arbeiten. 

Morphologisches  Jahrbuch. 

Museum  of  Comparative  Zoology.     Bulletin. 

Quarterly  Journal  of  Microscopical  Science. 

Royal  Society-  of  London,  Philosophical  Transactions. 

Zeitschrift  fiir  wissenschaftliche  Zoologie. 

Zoologischer  Anzeiger. 

Zoologischer  Jahrbiicher,  Abteilung  fiir  Anatomie  und  Entwicklungsgeschichte. 

Zoological  Society  of  London,  Proceedings  and  Transactions. 

TEXT -BOOKS,  MANUALS  AND  GENERAL  WORKS. 

Balfour:  Treatise  on  comparative  embr}-ology.     2  vols.,  London,  1880-82. 

Barker:  Anatomical  Terminology.  Philadelphia,  1907.  (Contains  nomenclature  of 
Basel  Commission — 'BNA.'). 

Bohm  und  Davidofif:  Histolog\%  trans,  by  Huber,  Philadelphia. 

Bronn's  Klassen  und  Ordnungen  des  Thierreichs. 

Works  of  John  Samuel  Budgett.  Cambridge,  1907.  (Mostly  teleosts,  dipnoi  and  am- 
phibia.) 

355 


356  BIBLIOGRAPHY. 

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system  bei  verschiededen  Wirbelthiere.     Anat,  Hefte,  13,  1910. 
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3—17,  1881-1904. 
Festschrift  zu  yosten  Geburtstage  Rudolf  Leuckarts.     Leipzig,   1892. 
Gegenbaur:    Vergleichende  Anatomic  der  Wirbelthiere.     2   vols.     Leipzig,    1898-1901. 
Handbuch  der  vergleichend.   und  experim.  Entwicklungslehre  (edited  by  O.  Hertwig). 

3  vols.,  Jena,  1901-1906. 
Hertwig:  Lehrbuch  der  Entwicklungsgeschichte  des  Menschen    und  der  Wirbelthiere, 

9th    edition,   Jena,   1910.     (An  earlier    edition,  trans,  by  Mark.     London,   1892.) 
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(Keibel   and   Abraham);    Ceratodus    (Semon);   rabbit    (Taylor);    Lacerta    (Peter); 

deer  (Keibel) ;  Tarsius  (Hubrecht) ;  man  (Keibel  und  Elze) ;  Acanthias  (Scammon) . 
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1892. 
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Oppel:  Lehrbuch    der    vergleichenden    mikroscopischen    Anatomie    der     Wirbelthiere. 

Jena,  1896 — Incomplete,  6  pts.  published. 
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and  reptiles  published)   Berlin,  1856.     (Invaluable  as  summary  of  older  work.) 
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(Abridgment  trans,  by  Parker,  London,  1908.) 
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adaptation  by  Eastman.     2  vols.     London.) 

MONOGRAPHS  ON  SINGLE  SPECIES  AND  GROUPS. 
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Cole:  Papers  on  anatomy  of  Myxine:  Trans.  Roy.  Socy.  Edinburg,  1905-12. 

Dean:  Development  of  Bdellostoma.     Quar.  Jour.  Micr.  Sci.,  40,  1897. 

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Scott,  W.  B.:  Entwicklimgsgeschichte  der  Petromyzonten.  Morphol.  Jahrb.,    7,  1882. 

Scott,  W.  B.r  Development  of  Petromyzon.     Jour.  Morph.,  i,  1887. 

Shipley.  A.:  Development  of  Petromyzon.     Quar.  Jour.  Micr.  Sci.,  27,  1887. 


GENERAL.  357 


Fishes. 


Allis:  Cranial  muscles  and  nerves  of  Amia.  Jour.  Morph.,  12,  1897;  of  Scomber,  same,  18, 

1903. 
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Jour.  Anat.  and  Physiol.,  1876-78.) 
Balfour   and   Parker:  Structure  and  development  of  Lepidosteus.     Phil.  Trans.,  1882. 
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Dean:  Development  of  garpike  and  sturgeon.     Jour.  Morph.,  11,  1895. 
Dean:  Chimaeroid  fishes  and  their  development.     Carnegie  Inst.,  1906. 
Garman,  S.:  Chlamydoselachus,  a  living  cladodont  shark.     Bull,  Mus.  Comp,  Zool.,  12. 
Gunther,  A.:  Ceratodus.     Philos.  Trans.  Royal  Soc'y.,  1871. 
Gunther:  Introduction  to  the  study  of  fishes.     Edinburgh,  1880. 
Kellicott:  Development  of  vascular  and  respirator}'  systems  of  Ceratodus.     Mem.  N.  Y. 

Acad.  Sci.,  2,  1905. 
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Rauther:  Panzerwelse.     Zool.  Jahrb.  Abt.  Anat.,  31,  1911. 

Wiedersheim:  Skclet  imd   Nervensystem  von   Lepidosiren.     Jena.   Zeitsch.,    14,  1892. 
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Hoffmann:  Amphibien,  in  Bronn's  Klassen  und  Ordnung  das  Thierreiches. 
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Sarasin,  P.  and  F.:  Entwicklungsgeschichte  urld  Anatomic  der  ceylonischen  Blind v^iihle 

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Wilder:  Anatomy  of  Siren  lacertina.     Zool.  Jahrbuch,  Abth.  Anat,,  4,  1891. 

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INTEGUMENT.  359 

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Jahrb.,  18,  1892. 
Mall:  Development  of  the  human  coelom.     Jour.  Morph.,  12,  1897. 
Mathes:  Morphologie    der    Mesenterialbildungen    bei    Amphibien.    Morph.  Jahrb.,  23, 

1895. 
Weber:  Abdominalporen  der  Salmoniden  nebst  Bemerkungen  iiber  Geschlechtsorgane. 

Morph.  Jahrb.,  12,  1887. 

INTEGUMENT. 

Batelli:  Bau  der  Reptilienhaut.     Arch.  mikr.  Anat.,  17,  1879. 

Boas:  V.  Wirbelthierekralle.     Morph.  Jahrbuch,  23,  1895. 

Boas:  Morphologie  der  Nagel,   Krallen,   Hufe  und   Klauen.     Morph.  Jahrb.,  11,  1884. 

See  also  21,  1894. 
Bonnet:  Die  Mammarorgane  (Summar)-),     Ergebnisse,  2,  1892;  7,  1898. 
Bonnet:  Haarspiral  und  Haarspindel.     Morph.  Jahrb.,  11,  1886. 
Bowen:  Epitrichium  of  human  epidermis.     Anat.  Anz.,  4,  1889. 
Brauer:  Leuchtorgane  der  Knochenfische.     Verhandl.  zool.  Gesellsch.,  13,  1904. 
Burckhardt:  Luminous  organs  of  selachians.     Ann.  and  Mag.  Nat.  Hist.,  VII,  6,  1900. 
Davies:  Entwickelung  der  Feder,  u.  s.  w.  Morph.  Jahrb.,  15,  1889. 
Drasch:  Giftdnisen  des  Salamanders.     Verhandl.  anat.  Gessellsch.,  6,  1892. 
Esterly:  Poison  glands  of  Plethodon.     Pubs.  Univ.  Gal.,  Zool.,  i,  1904. 
Goppert:  Phylogenie  der  Wirbelthierkralle.     Morph.  Jahrb.,  25,  1898. 
Green:  Phosphorescent  organs  in  Porichthys.  Jour.  Morphol.,     15,  1900. 
Henneberg:  Entwicklung  der  Mammarorgane  der  Ratte.     Anat.  Hefte,  13,  1899. 
Japha:  Haare  der  Waltiere.     Zool.  Jahrb.,  Abt.  Anat.,  32,  1911. 
Jeffries:  Epidermal  system  of  birds.     Proc.  Boston  Socy.  Nat.  Hist.,  22,  1883. 
Jones:  Development  of  the  nestling  feather.     Oberlin  Lab.  Bulletin,  13,  1907. 
Kerbert:  Haut  der  Reptllien.     Archiv  mikr.  Anat.,  13,  1876. 
Keibel:  Ontogenie  und  Phylogenie  von  Haare  und  Feder.     Ergebnisse,  5,  1895. 


360  BIBLIOGRAPHY. 

Kerr:  Development  of  skin  and  derivatives  in  Lepidosiren.     Quar.  Jour.  Micros.,  Sci.,  46, 

1902. 
Krause:   Entwickelung  der    Epidermis  und  ihrer  Nebenorgane.     Handbuch   der    En- 

twicklungslehre,  2,  Th.  i,  1902. 
von  Lendenfeld:  Phosphorescent  organs  of  fishes.     Challenger  reports,  22,  1S87.     Sum- 
mary in  Biol.  Centralblatt,  7,  1887. 
Leydig:  Vascularisiertes  Epithel.     Archiv  mikr.  Anat.,  52,  1898. 
Maurer:  Vascularisierung  der  Epidermis  bei  Anuren.     Morph.  Jahrb.,  26,  1898. 
Maurer:  Haut-Sinnesorgane,  Feder,  und  Haaranlagen,  Morph.  Jahrb.,  18,  1892;  29,  1893 

See  also  26,  1898. 
de  Meijere:  Die  Haare  der  Saugetiere  imd  ihre  Anordnung.     Morph.  Jahrb.  21,  1894, 

Feathers,  23,  1895. 
Muhse:  Cutaneous  glands  of  toad.     Am.  Jour.  Anat.,  9,  1909. 
Nicoglu:  Hautdriisen  der  Amphibien.     Zeitschr.  wiss.  Zool.,  56,  1893. 
O'Donoghue:  Growth  changes  of  mammary  apparatus  of  Dasyurus.     Quar.  Jour.  Micr 

Sci.,  57,  1911. 
Parker:  Movement  of  melanophore  pigment,  especially  in  lizards.     Jour.  Exp.  Zool.,  3, 

1906. 
Parker  and  Starrall:  Color  changes  in  skin  of  Anolis.     Proc.  Am.  Acad.  A,  and  S.,  40, 

1906. 
Poulton:  Bill  and  hair  of  Ornithorhynchus.     Quarterly  Jour.  Micro.  Sci.,  36,  1894. 
Profe:  Ontogenie  und  Phylogenie  der  Mammarorgane.     Anat.  Hefte,  11,  1898. 
Pycraft:  Interlocking  of  feathers.     Nat.  Science,  3,  1893. 
Reed:  Poison  glands  of  catfishes.     Amer.  Nat.,  41,  1907. 
Reh:  Schuppen  der  Saugetiere.     Jena.  Zeitsch.,  29,  1895. 
Romer:  Schuppen  und  Haare  am  Schwanz  von  Mus.  Jena.  Zeit.,  30,  1896.     See  also  31, 

1898. 
Sawadsky:  Larval  Haftapparates  bei  Acipenser.     Anat.  Anz.,  40,  191 1. 
Schmidt:  Integument  von  Voeltzkowia  (lizard).     Zeit.  wiss.  Zool.,  94,  1910. 
Schulz:  Giftdriisen  der  Krote  und  Salamander.     Archiv  mikr.  Anat.,  34,  1889. 
Schwalbe:  Farbenwechsel  winterweisser  Thiere.     Morph.  Arbeiten,  2,  1893. 
Semon:  Mammarorgane  der  Monotremen.     Morph.  Jahrb.,  28,  1899. 
Spencer  and  Sweet:  Hairs  of  Monotremes.     Quar.  Jour.  Mic.  Sci.,  41,  1899. 
Stohr:  Entwicklimg  des  menschlichen  Wollhaares.     Anat.  Hefte,  23,  1903. 
Strong:  Development  of   the  primitive  feather.     Bull.  Mus.  Comp.   Zool.,   40,    1902. 
Wallace:  Axillary  gland  of  Batrachus.     Jour.  Morphol.,  8,  1893. 
Whipple:  Naso-labial  groove  of  lungless  salamanders.     Biol.  Bull.,   11,   1906. 
Wilder:  Palms  and  soles.     Amer.  Jour.  Anat.,  i,  1902. 
Zeitschmann:  (Odoriferous  glands  of  Cervidae).     Zeits.  wiss.  Zool.,  74,  1903. 

DERMAL  SKELETON. 

Boldt:  Ruckenschild  der  Ceratophrys.     Zool.  Jahrb.  Abt.  Anat.,  32,  1911. 

Hase:  Schuppenkleid  der  Teleostier.      Jena.  Zeitsch.,  42,  1907. 

Hase:  Morphol.  Entwicklung  der  Ktenoidschuppen.     Anat.  Anz.,  40,  191 1. 

Hertwig:  Placoidschuppen  und  Zahne  der  Selachier.     Jena.  Zeitsch.,  8,  1874. 

Hertwig:  Hautskelett  der  Fische.     Morph.  Jahrb.,  2,  1876;  5,  1879;  7,  1881. 

Klaatsch:  Morphologic  der  Fischenschuppen.     Morph.  Jahrb.,  16,  1890. 

Nickerson:  Development  of  scales  of  Lepidosteus      Bull.  Mus.  Comp.  Zool.,  24,  1893. 

Parker:  Correlated  abnormalities  in  scutes  and  plates  of  tortoise.     Amer.  Nat.,  35,  1901. 

Romer:  Panzer  der  Giirteltiere.     Jena.  Zeitsch.,  27,  1892. 

Tims:  Development  and  morphology  of  scales  of  some   teleosts.     Quar.  Jour.  Micros. 

Sci.,  49,  1905. 
Wiedersheim:  Histologie  der  Dipnoerschuppen.     Arch.  mikr.  Anat.,  18,  18S0. 


SKELETON.  361 

SKELETON— GENERAL. 

Baur:  Morpholog}*  of  ribs.     Amer.  Naturalist,  21,  1887.     Jour.  Morph.,  3,  1889.     (See 

Anat.  Anz.,  9,  1883.) 
Baur:  Carpus  and  Tarsus  der  Wirbeltiere.     Zool.  Anz,,  8,  1885. 
Braus:  Gliedmassenpropfung  und  Grundfragen  der  Skeletbildung.     Morph.  Jahrb.,  39, 

1909. 
Cope:  \^ertebrates  of  the  tertiary  formations  of  the  West.     Report  U.  S.  (Hayden's)  Geol. 

Survey,  3,  1884. 
Dean:  Origin  of  paired  limbs.     Amer.  Nat.,  36,  1902. 
Gadow:  First  and  second  visceral  arches  with  reference  to  the  homologies  of  the  auditory 

ossicles.     Phil.  Trans.,  179,  1888. 
Gaupp:  Schalleitenden  Apparates  bei  den  Wirbeltiere,     Ergebnisse,  8,  1899. 
Gaupp:  Schlafendgegend  am  Wirbeltiereschadel.     Morph.  Arbeit,,  4,  1894. 
Gaupp:  Alte  Probleme  und  neuere  Arbeiten  iiber  Wirbeltiereschadel.     Ergebnisse,  10, 

1900. 
Gaupp:  Fragen  an  d.  Lehre  vom  Kopfskelett.     Anat.  Anz.,  Erganzimgshefte,  29,  1896. 
Gaupp:  Unterkiefer  der  Wirbeltiere.     Anat.  Anz.,  39,  191 1. 

Gaupp:  Saugerpter\'goid  imd  Echidnapter}'goid,  u.  s.  w.     Anat,  Hefte.,  42,  1910, 
Gegenbaur:  Skelett  der  Gliedmassen  der  Wirbelthiere  und  der  Hintergliedmassen  der 

Selachier.     Jena.  Zeitsch.,  5,  1870. 
Gegenbaur:  Uber  das  Archipterjgium.     Jena.  Zeitsch.,  7,  1872. 
Gegenbaur:  Primare  imd  secundare  Knochenbildung.     Jena.  Zeitsch.,  3,   1867. 
G^enbiur:  Untersuchungen  zur  vergl.  Anatomie  der  Wirbelthiere.     3  parts,  Leipzig,  1872. 
Gotte:  Morphologie  des  Skelettsystems  der  Wirbeltiere.     (Girdle  and  breastbone)  Arch. 

f.  nukr.  Anat.,  15,  1878. 
Hertwig:  Zahnsystem  der  Amphibien  und  seine  Bedeutimg  fiir  die  Genese  des  Skeletts  der 

Mundhohle.     Arch.  mikr.  Anat.,  11,  1874. 
Howes:  Morphology  of  the  sternum.     Nature,  43,  1891. 
Kingsley:  The  ossicula  auditus.     Tufts  Coll.  Studies,  i,  1900. 
Parker  and  Bettany:  Morphology  of  the  skull,     London,  1877. 
Parker:  Structure  and  development  of  the  shoulder  girdle  and  sternum.     Rav  Societj', 

1867. 
Parker  and  Bettany:  Morpholog}'  of  the  skull.     London,  1877. 
RejTiolds:  Vertebrate  skeleton.     Cambridge  (England),  1897. 
Thacher:  Median  and  paired  fins.     Trans.  Conn.  Acad,,  3,  1877, 
Thyng:  Squamosal  bone  in  Tetrapoda,     Tufts  Coll.  Studies,  2,  1906. 
Woodward:  Vertebrate  paleontolog\'.     Cambridge,  1898. 
Wiedersheim:  Gliedmassenskelett  der  Wirbelthiere.     Jena,  1892. 
Winslow:  Chondrocranium  in  the  ichthyopsida.     Tufts  Coll.  Studies,  i,  1898. 
Zittell:  Handbuch  der  Palaontologie.     Miinchen,  1884-93.     (A  translation  of  an  abridg- 
ment, by  Eastman,  London,  1900.) 

Fishes. 

Adams:  Skull  of  Anarrhichthys.  Kansas  Univ.  Sci.  Bull.,  4,  1908. 
Allis:  Cranium  of  Amia.     Jour.  Morph.,  2,  1889;  14,  1898. 
Allis:  Jaw  bones  and  breathing  valves  of  Polypterus.     Anat.  Anz.,  18,  1900. 
Allis:  Skull  of  Scomber.     Jour.  Morph.,  18,  1903, 

Balfour:  Development  of   skeleton  of  paired  fins  of  elasmobranchs.     Proc.  Zool.  Socy. 
London,  1881. 

Bridge:  Ribs  in  Polyodon.     Proc.  Zool.  Socy,  London,  1897, 

Brohmer:  Kopf  eines  embryos  von  Chlamydoselachus  u,  d,  Segmentirung  des  Selachier- 
schadels,     Jena,  Zeitsch.,  44,  1908.     : 


362  BIBLIOGRAPHY. 

Boyer:  Mesoderm  in  teleosts  and  its  share  in  pectoral  fin.     Bull.  Mus.  Comp.  Zool.,  23, 

1892. 
Cartier:  Entwicklungsgeschichte  der  Wirbelsaule.     Zeitsch.  wiss.  Zool.,  25,  1875. 
Corning:  Urwirbelknospen  in  der  Brustflossen  der  Teleostier.     Morph.  Jahrb.,  22,  1894. 
Cramer:  Skull  of  Sebastodes.     Stanford  Univ.  Pub.,  2,  1895. 
Dean:  Cladoselache.     Jour.  Morph.,  9,  1894. 
Dean:  Fin-fold  origin  of  paired  fins.     Anat.  Anz.,  11,  1896. 
Derjugin:  Entwicklung  des  Schultergurtels  und  Brustflosse  bei  den  Teleostiern.     Zeit. 

wiss.  Zool.,  96,  1910. 
Dohrn:  Paarigen  und  unpaarigen  Flossen  der  Selachier.     Naples  Mittheilungen,  5,  1886. 
Foote:  Extrabranchials  in  elasmobranchs.     Anat.  Anz.,  13,  1897. 
Flirbringer:  Visceralskelett  der  Selachier.     Morph.  Jahrb.,  31,  1903. 
Gadow  and  Abbott:  Evolution  of  vertebral  column  of  fishes.     Phil.  Trans.  186B,  1895. 
Gaupp:  Entwicklung   der  Schadelknocken  b.  d.  Teleostiern.     Anat.  Anz.,  Erganzung- 

shefte  zu  23,  1903. 
Gegenbaur:  Entwicklungsgeschichte  der  Wirbelsaule  des  Lepidosteus.     Jena.  Zeitschr., 

3,  1867. 
Gegenbaur:   Untersuchungen  zu  vergleichenden  Anatomie  der  Wirbelthiere.     Leipzig, 

1872.     (Skull  of  elasmobranchs,  girdles  of  fishes.) 
Gegenbaur:  Kopfskelett  von  Alepocephalus.     Morph.  Jahrb.,  4,    Suppl.,  1878. 
Gegenbaur:  Clavicula  und  Cleithrum.     Morph.  Jahrb.,  23,  1895. 
Gegenbaur:  Flossenskelett   der   Crossopterygier   und    das   Archipterygium   der  Fische. 

Morph.  Jahrb.,  22,  1894. 
Gegenbaur:  Uber  das  Archipterygium.     Jena.  Zeitsch.,   7,  1872. 

Goodrich:  Pelvic  girdle  and  fin  of  Eusthenopteron.     Quar.  Jour.  Micr.  Sci.,  45,  1901. 
Goodrich:  Dermal  finrays  of  fishes.     Quar.  Jour.  Micr.  Sci.,  47,   1904. 
Goodrich:  Development,  structure  and  origin  of  fins  of  fishes.     Quar.  Jour.  Micr.  Sci., 

50,  1906. 
Goppert:  Morphologie  der  Fischrippen.     Morph.  Jahrb.,  23,  1896. 
Grassi:  Entwicklung  der  Wirbelsaule  der  Teleostier.     Morph.  Jahrb.,  8,  1882. 
Harrison:  Entwickelung  der  unpaaren  und  paarigen  Flossen  der  Teleostier.     Arch.  mikr. 

Anat.,  46,  1895.    .See  also  42,  1893. 
Hasse:  Entwicklung  der  Wirbelsaule  der  Elasmobranchier.     Zeitsch.  wiss.  Zool.,  55,  1892; 

Ganoiden,  57,  1893;  Cyclostomen,  57,  1893. 
Hay:  Vertebral  column  of  Amia.     Field  Columbian  Museum,  Zool.  Publications,  i,  1895 
Klaatsch:  Vergleich.  Anat.  der  Wirbelsaule.     Morph.  Jahrb.,  19-22,  1893-95. 
Mayer:  Unpaaren  Flossen  der  Selachier.     Naples  Mittheilungen,  6,  1885. 
Parker:  Structure  and  development  of  the  skull  in  sharks  and  skates.     Trans.  Zool.  Socy. 

London,  10,  1878. 
Parker:  Skeleton  of  Marsipobranchs.     Phil.  Trans.,  1883. 

Parker:  Structure  and  development  of  skull  in  Lepidosteus.     Phil.  Trans.,  1882;  of  stur- 
geons, same  volume. 
Pollard:  Suspension  of  jaws  in  fishes.     Anat.  Anz.,  10,  1894. 

Pollard:  Oral  cirri  of  siluroids  and  origin  of  head.  Zool.  Jahrb.,  Anat.  Abth.,  8,  1895. 
Sagemehl:  (several  papers  on  skulls  of  fishes).  Morph.  Jahrb.,  9,  10,  17,  1884-1891. 
Shufeldt:  Osteology  of  Amia  (based  on  Francque).     Rept.  U.  S.  Fish  Commis.  for  1883 

1884. 
Starks:  Several  papers.    Proc.  U.  S.  Nat.  Mus.,  21-27,  1898-1904. 
Thacher:  Median  and  paired  fins.  Trans.  Conn.  Acad.,  3,  1878. 
Thacher:  Ventral  fins  of  ganoids.     Trans.  Conn.  Acad.,  4,  1878. 

Veit:  Entwicklung  des  Primordialcranium  von  Lepidosteus.  Anat.  Hefte.,  44,  191 1. 
Walther:  Entwicklung  der  Deckknochen  am  Kopfskelett  des  Hechtes.     Jena.  Zeitsch.,  16, 

1883. 


SKELETON.  363 

\\right:  Skull  and  auditory  organ  of  Hypophthalmus,     Trans.  Roy.  Soc.  Canada,  4,  1885. 
Ziegler:  Hornfaden  der  Selachier  und  die  Floss  en  strahlen  der  Knochenfische.     Zool.  Anz., 
33,  190S. 

Amphibia. 

Cope:  Hyoid  and  otic  elements  in  batrachia.     Jour.  Morph.,  2,  1888. 

Credner:  Stegocephalen  und  Saurier.     Zeitschr.  deutsch.  geolog.     Gesellsch,,  1881-1893. 

Eggeling:  Aufbau  der  Skeletteile  in  Gliedmassen.     Jena,  191 1. 

Field:  Entwickelung  der  Wirbelsaule  der  Amphibien.     Morph.    Jahrb.,   22,    1895. 

Gadow:  Evolution  of  vertebral  column  of  amphibia  and  amniotes.     Phil.  Trans.,  187,  B, 

1896. 
Gaupp:  Primordial-cranium  von  Rana.     Morph.  Arbeiten,  2,  1893. 
Goppert:  Amphibienrippen.     Morph.  Jahrb.,  22,  1895. 

Kingsbur}'  and  Reed:  Columella  auris  in  amphibia.     Jour.  Morph.,  20,  1909. 
-Murray:  \'ertebral  column  of  certain  urodeles.     Anat.  Anz.,  13,  1897. 
Parker:  Structure  and  development  of  skull  in  urodeles.     Phil.  Trans.,  1877. 
Parker:  Morpholog}'  of  skull  in  Amphibia  urodela.     Trans.  Linn.  Socy.,  Zool.,  2,  1879. 
Parker:  Structure  and  develop,  of  skull  of  common  frog.     Phil.  Trans.,  187 1. 
Parker:  Struct,  and  dev.  skull  in  Batrachia,  Pt.  II  and  III.     Phil.  Trans.,  1881 
Parker:  Struct,  and  dev.  skull  in  urodeles.     Trans.  Zool.  Socy.  London,  11,  1882. 
Peter:  Schadel  von  Ichthyophis.     Morph.  Jahrb.,  25,  1898. 

Piatt:  Development  of  cartilaginous  skull  of  Necturus.     Morph.  Jahrb.,  25,  1897. 
Shimada:  Wirbelsaule  und  Hiillen  des  Riickenmark  von  Cryptobranchus  japonicus.     Anat. 

Hefte,  44,  191 1. 
Terry:  Xasal  skeleton  of  Amblystoma.     Trans.  St.  Louis  Acad.  Sci.,  16.  1906. 
Whipple:  Ypsiloid  apparatus  of  urodeles.     Biol.  Bull.,  10,  1906. 
Wiedersheim:  Kopfskelett  der  Urodelen.     Morph.  Jahrb.,  3,  1877. 
Wilder:  Skeletal  system  of  Necturus.     Memoirs  Boston  Socy.  Nat.  Hist.,  5,  1903. 

Reptilia. 

Baur:  Osteologische  Notizen  iiber  Reptilien.     Zool.  Anz.,  9  and  10,  1886-87. 

Baur:  Pelvis  of  the  Testudinata.     Jour.  Morph.,  4,  1891. 

Baur:  Morphologie  des  Carpus  und  Tarsus  der  Reptilien.     Zool.  Anz.,  8,  1885. 

Cope:  Osteolog}'  of  lacertilia.     Proc.  Am.  Phil.  Socy.,  30,  1892. 

Corning:  Neugliederung  der  Wirbelsaule  bei  Reptilien.     Morph.  Jahrb.,  17,  189 1. 

Cope:  Degenerate  scapular  and   pelvic  arches  in   lacertilia.     Jour.   Morph.,   7,    1892. 

Fiirbinger:  Knochen  und  Muskeln  der  Extremitaten  bei  schlangenahnlichen  Saurien. 

Leipzig,  1S70, 
Gaupp:  Chondrocraniun  von  Lacerta.     Anat.  Hefte,  15,  1900. 
Gotte:  Wirbelbaubei  ReptiUen.     Zeitsch.  wiss.  2k>ol.,  50,  1892. 
Gotte:  Entwicklung des  Carapax  der  Schildkroten.     Zeit.  wiss.  Zool.,  66,  1899. 
Hay:  Fossil  turdes  of  North  America.     Carnegie  Inst.,  1908. 
Howes   and  Swinnerton:  Development  of  skeleton  of  Sphenodon.     Trans.  Zool.  Socy. 

London,  16,  1901. 
Kingsley:  Reptilian  lower  jaw.     Amer.  Nat.,  39,  1905. 
Marsh:  Numerous  papers  on  fossil  reptiles.     Am.  Jour.  Sci.,  16-50. 
Moodie:  Reptilian  epiphyses.     Am.  Jour.  Anat.,  7,  1908. 
Neit:  Schadel  d.  Dermochelys.     Zool.  Jahrb.,  Abt.  Anat.,  7,1,,  1912. 
Ogushi:  Skelett  der  japanischen   Trionyx.     Morph.  Jahrb.,  43,  191 1. 
Parker:  Skull  in  the  common  snake.     Phil.  Trans.,  1878. 
Parker:  Skull  in  the  lacertilia.     Phil.  Trans.,  1879. 


364  BIBLIOGRAPHY. 

Parker:  Skull  in  the  crocodile.     Trans.  Zool.  Socy.,  19. 

Versluys:  Mittlere  und  aussere  Ohrsphare  der  Lacertilia.     Zool.  Jahrb.,  Abth.  Anat.,  12, 

1898. 
Versluys:  Columella  auris  bei  Lacertilien.     Zool.  Jahrb.,  Abth.  Anat.,  19,  1903. 

Birds. 

Gegenbaur:  Becken  der  Vogel.     Jena.  Zeitsch.,  6,  18,  1871. 

Leighton:  Development  of  wing  of  Sterna.     Tufts  Coll.  Studies,  i,  1894. 

Lindsay:  The  avian  sternum.     Proc.  Zool.  Socy.  London,  1885. 

Marsh:  Odontornithes.     U.  S.  Geol.  Survey,  1880. 

Mehnert:  Entwicklung  des  Os  pelvis  der  Vogel.     Morph.  Jahrb.,  13,  1888. 

Morse:  Carpus  and  tarsus  of  birds.     Ann.  N.  Y.  Lyceum  Nat.  Hist.,  10,  1872. 

Osborn:  Evidence  for  a  dinosaur-avian  stem  in  the  Permian,,  Am.  Nat.,  34. 

Parker:  Skull  of  the  common  fowl:  Phil.  Trans.,  1869. 

Pycraft:  Osteology  of  birds.     Proc.  Zool.  Socy.  London,  1898. 

Parker:  Morphology  of  duck  and  auk  tribes.     Royal  Irish  Acad.,  Cunningham  memoir, 

6,  1890. 
Parker:  Structure  and  development  of  wing  in  common  fowl.     Phil.  Trans..  179,  1888. 
Shufeldt:  Osteology  of  birds.     Bulletin  N.  Y.  State  Museum,  130,  1909.     (Gives  full  list 

of  the  author's    numerous  papers  on  avian  anatomy.) 
Siegelbauer:  Entwicklung  der  Vogelextremitat.     Zeit.  wiss.  Zool.,  97,  1911. 

Mammals. 

Allen:  Ethmoid  bone  in  mammals.     Bull.  Mus.  Comp.  Zool.,  10,  1883. 

Bardeen:  Development  of  human  skeleton.     Am.  Jour.  Anat.,  4,  1905. 

Bardeen:  Development  of  thoracic  vertebrae  in  man.     Jour.  Anat.,  4,  1905. 

Baur:  Morphologic  des  Carpus  der  Sauger.     Anat.  Anz.,  4,  1889. 

Brush:  Cervical  ribs.     J,  Hopkins  Hosp.  Bull.,  12,  1901. 

Boas:  Metatarsus  der  Wiederkauer.     Morph.  Jahrb.,  16,  1890. 

Collinge:  Skull  of  dog.     London,  1896. 

Dilg:  Morphologie  des  Schadels  bei  Manatus.     Morph.  Jahrb.,  39,  1909. 

Eggeling:   Clavicula,  Praeclavicula,  Halsrippen  und  Manubrium  stemi.     Anat.  Anz.,  29, 

1906. 
Fischer:  Primordialcranium  von  Talpa.     Anat.  Hefte,  17,  1901. 
Flower:  Osteology  of  the  mammalia.     London,  1885. 

Frits:  Entwicklung  der  Wirbelsaule  von  Echidna,     Morph.  Jahrb.,  39,  1909. 
Froriep:  Entwicklung  der  Wirbelsaule.     Arch.  An?t.  und  Phys.,  1886. 
Gaupp:  Neue  Deutungen  a.  d.  Gebiet  der  Lehre  des  Saugetierschadels.     Anat.  Anz.,  27, 

1905. 
Gegenbaur:  Epistemal  Skelettteile.     Jena.  Zeitsch.,  i,  1864. 
Holder:  Osteology  of  right  whale.     Bull.  Am.  Mis.  Nat.  Hist.,  i,  1883. 
Howes:  Mammalian  hyoid.     Jour.  Anat.  and  Phys.,  30,  1897. 
Kiikenthal:  Hand  der  Cetaceen.     Anat.  Anz.,  3,  1888;  Morph.  Jahrb.,  19,  1S92. 
Mead:  Chondrocranium  of  pig.     Am.  Jour.  Anat.,  9,  1909. 
Olmstead:  Primordialcranium  eines  Hundeembryo.     Anat.  Hefte,  43,  191 1. 
Parker:  Skull  of  pig.     Phil.  Trans.,  1874. 
Parker:    Structure   and   devel.  skull  in  mammalia,  Pt.  2,  Insectivora.  Pt.  3,  Edentates. 

Phil.  Trans.,  1885. 
Stromer:  Foramen  entepicondyloideum  und  Trochanter  tertius.     Morph.  Jahrb.,  29, 1902, 
Van  Kampen:  Tympanalgegend  des  Saugetierschadels.     Morph.  Jahrb.,  34,  1905. 
Voit:  Primordialcranium  des  Kaninchens.     Anat.  Hefte,  38,  1909. 


MUSCULAR  SYSTEM.  365 

Weiss:  Entwicklung  der  Wirbelsaule  der  weissen  Ratte.     Zeitsch.  wiss.,  Zool.,  69,  1901. 
Whitehead  and  Waddell:  Development  of  human  sternum.     Am.  Jour.  Anat.,  12,  191 1. 

MUSCULAR  SYSTEM. 

AlUs:  Cranial  muscles  of  Amia.     Jour.  Morph.,  12,  1897. 

Avers  and  Jackson:  Myolog\'  of  myxinoids.     Jour.  Morph.,  17,  1901. 

Blum:  Schwanzmuskulatur  des  Menschen.     Anat.  Hefte,  4,   1894. 

Braus:  Entwicklung  der  Musculatur  und  periph.  Nervensystem  der  Selachier.     Morph. 

Jahrb.,  26,  27,  1898-9. 
Bruner:  Smooth  facial  muscles  of  Amphibia.     Morph.  Jahrb.,  29,  1901. 
Byrnes:  Develop,  limb  muscles  in  amphibia.     Jour.  Morph.,  14,  1897. 
Chamock:  Muscles  of  mastication  and  movements  of  skull  in  lacertilia.     Zool.  Jahrb., 

Anat.  .\bth.,  18,  1903. 
Coming:  Entwicklung    Kopf-    und    Extremitaten-Muskulatur    bei    ReptiUen.     Morph. 

Jahrb..  2S,  1899. 
Coming:  Vergl.  Anat.  der  Augenmuskulatur.     Morph.  Jahrb.,  29,  1900. 
Davidoff:  ^'e^gl.   Anat.   der  hinteren   Gliedmassen   der  Fische.     Morph.    Jahrb.,    5-6, 

1879-So. 
Driiner:  Zungenbein-,  Kiemenbogen-  und  Kehlkopfmuskeln  der  Urodelen.     Zool.  Jahrb. 

Abth.  Anat.,    15,  1901;  19,  1904. 
Edgeworth:  Development  of  head  muscles  in  Gallus.     Quar.  Jour.  Micr.  Sci.,  51,  1907. 
Edgeworth:  Morpholog)^  of  the  cranial  muscles  of  some  vertebrates.     Quar.  Jour.  Micr. 

Sci. J  56,  191 1. 
Fewkes:  Myolog)'  of  Echidna.     Bull.  Essex  Inst.,  9,  1877. 
Fiirbringer:  Muskeln  der  schlangenahnlichen  Saurien.     Leipzig,  1870. 
Fiirbringer:  \ergl.  Anat.  der  Schultermu skein.     Urodeles,  Jena.  Zeitsch.,  7,  1873;  Anura, 

1.  c,  8,  1S74;  Birds,  1.  c,  36,  1902;  Reptiles.     Morph.  Jahrb.,  i,  1876. 
Fiirbringer:  Muskulatur  des  Kopf  der  Cyclostomen.     Jena.  Zeitsch.,  9,  1875. 
Fiirbringer:  Muskulatur  des  Vogelfliigels.     Morph.  Jahrb.,  6,  1885. 
Gadow:  Bauchmuskeln  der  Reptilien.     Morph.  Jahrb.,  7,  1881. 
Gadow:  Myologie  der  Extremitaten  der  Reptilien.         Morph.   Jahrb.,  7,  1881. 
Humphrey:  Several  papers  on  muscles  of  sharks,  dipnoi  and  urodeles.     Jour.  Anat.  and 

Phys.',  1873. 
Humphrey:  Muscles  of  Lepidosiren   (Protopterus) .     Jour.  Anat.  and  Phys.,  6,   1872. 

Muscles  of  Ceratodus,  same  vol. 
Lamb:  Eye-muscles  in  Acanthias.     Am.  Jour.  Anat.,  i,  1902. 
MacDowell:  Myology  of  Anthropopithecus.     Am.  Jour.  Anat.,  10,  19 10. 
McMurrich:  Phylogeny  of  forearm  flexors.     Am.  Jour.  Anat.,  2,  1903;  of  palmar  mus- 
culature, same  vol. 
Mall:  Development  of  human  diaphragm.     Jour.  Morph.,  12,  1897. 
Mall:    Development    of   human   diaphragm.     Johns   Hopkins   Hosp.  Bull.,    12,    1901. 
Marion:  Mandibular  and  branchial  muscles  of  elasmobranchs.     Am.  Nat.,  39,   1905; 

Tufts  College  Studies,  2,  1905. 
Maurer:  \'entral  Rumpfmuskulatur  der  Urodelen.     Morph.  Jahrb.,  17,  1892. 
Mivart:  Myology  of  Menopoma,  Menobranchus,  Chameleon.     Proc.  Zool.  Socy.  London, 

1869-70. 

Xeal:  Development  of  hypoglossal  musculature  in  Petromyzon  and  Squalus.     Anat.  Anz., 
13,  1S97. 

Ribbing:  Armmuskulatur  der  Amphibien,  Reptilien  und  Saugetiere.     Zool.  Jahrb.  Abth. 

Anat.,  II,  1907. 
Ruge:  Geschichtsmuskeln  der  HalbafiFen.     Morph.  Jahrb.,  11,  1885. 
Schufeldt:  Myology  of  the  raven.     London,  1890. 
Shufeldt:  Anatomy  (mostly  muscles)  of  Geococcyx.     Proc.    Zool.  Socy.  London,  1886. 


366  BIBLIOGRAPHY. 

Uskow:  Entwicklung  des  Zwergfells,  u.  s.  w.  Arch.  mikr.  Anat.,  32,  1883. 
Wilder:  Appendicular  muscles  of  Necturus.     Zool.  Jahrb.,  Suppl.  15,  2  Ed.,  1912. 

ELECTRICAL  ORGANS. 

Ballowitz:  Anatomic  des  Zitteraales.     Arch.  mikr.  Anat.,  50,  1897. 

Ballowitz:   Elektrischen  Organe  von  Torpedo.  Arch.  mikr.    Anat.,     42,     1893     (Large 

bibliography.) 
Dahlgren  and  Silvester:  Electric  organ  of  Astroscopus.     Anat.  Anz.,  29,  1906. 
Ewart:  Development  of  electric  organ  in  skate.     Phil.  Trans.,  179B,  1889. 

NERVOUS  SYSTEM. 

Barker:  The  nervous  system  and  its  constituent  neurones.     N.  Y.  ,  1899. 

Edinger:  Vorlesungen  iiber  den  Bau  der  nervosen  Centralorgane  des  Menschen  und  der 

Tiere.     7th  edit.,  2  vols.,  1904-8. 
Johnston:  Nervous  system  of  vertebrates.    Philadelphia,  1906. 
Johnston:  Morphology  of  vertebrate    head    from  viewpoint  of    functional  divisions  of 

nervous  system.     Jour.  Comp.  Neurol.,  15,  1905. 
Johnston:  Central   nervous    system   of  Vertebrates.     Ergebnisse   und   Fortschritte   der 

Zoologie,  2,  1910. 

BRAIN  AND  SPINAL  CORD. 

Barnes:  Development  of  posterior  fissure  of  spinal  cord.     Proc.  Am.  Acad.  A.  and  Sci., 

1883-4. 
Dejerine:  Anatomic  des  centres  nerveux.     Paris,  1895. 
Herrick:  Morphology  of  forebrain  in  amphibia  and  reptiles.     Jour.   Comp.  Neurol.,  20, 

1910. 
Hill:  Primary  segments  of  vertebrate  head.     Zool.  Jahrb.,  13,  1899. 

His:  Allgemein.  Morphologic  des  Gehirn.  Arch.  Anat.  und  Phys.,  Abth.  Anat.,  1892. 
Johnston:  Morphology  of  forebrain  vesicle  in  vertebrates.  Jour.  Comp.  Neurol.,  19,  1909. 
Johnston:  Morphology  of  vert,  head  from  point  of  division  of  nervous  system.     Jour. 

Comp.  Neurol.  15,  1905. 
Johnston:  Gehirn  und  Cranialnerven  der  Anamnier.     Ergebnisse,  11,  1901. 
McClure:  Segmentation  of  primitive  brain.     Jour.  Morph.,  4,  1890. 
Nakagawa:  Origin  of  cerebral  cortex  and  homology  of  optic  lobe  layers.     Jour.  Morph., 

4,  1890. 
Osborn:  Origin  of  corpus  callosum.     Morph.  Jahrb.,  12,  1886. 
Smith:  Origin  of  corpus  callosum.     Trans.  Linn.  Socy.,  7,  1897. 
Tilney:  Hypophysis  cerebri.     Memoirs  Wistar  Inst.,  2,  191 1. 

Cyclostomes  and  Fishes. 

Ayers  and  Worthington:  Finer  anatomy  of  brain  of  Bdellostoma.     Am.  Jour.  Anat.,  8, 

1908. 
Bing  und  Burckhardt:  Zentralnervensystem  von  Ceratodus.     Anat.  Anz.,  25,  1904. 
Burckhardt:  Centralnervensystem  von  Protopterus.     Berlin,  1892. 
Chandler:  Lymphoid  structure  above  myelencephalon  of  Lepidosteus.  •  Univ.  Calif.  Pub. 

Zool.,  9,  191 1. 
Cole:  Cranial  nerves  of  Chimaera.     Trans.  Roy.  Socy.  Edinb.,  38,  1896. 
Dammerman:  Der  Saccus  vasculosus  der  Fische  ein  Tieforgane.     Zeit.  wiss.  Zool.,  96, 

1910. 
Franz:  Das  Mormyriden  (brain).   Zool.  Jahrb.,  Abt.  Anat.,  32.  191 1. 
Franz:  Kleinhirn  der  Knochenfisrhe.  Zool.  Jahrb.,  Abt.  Anat.,  32,  191 1. 


NERVOUS  SYSTEM.  367 

Goronowitsch:  Gehim  und  Cranialnerven  von  Acipenser.     Morph.  Jahrb.,  13,  1888. 

Haller:  Bau  der  Wirbeltiergehixns.  I,  Salmo  und   Scyllium.     Morph.  Jahrb.,  26,    1898. 

Herrick:  Brains  of  some  American  fresh  water  fishes.     Jour.  Comp.  Neurol.,  i,  1891. 

Herrick:  Brain  of  certain  ganoids.     Jour.  Comp.  Neurol.,  i,  1891. 

Johnston:  Brain  of  Acipenser.     Zool.  Jahrb.,  15,  1901, 

Johnston:  Brain  of  Petromyzon.     Jour.  Comp.  Neurol,,  12,  1902. 

Johnston:  Telencephalon  of  selachians.     Jour.  Comp.  Neurol.,  21,  191 1. 

Johnston:  Olfactory  lobes,  forebrain  and  habenular  tracts  of  Acipenser.     Zool.  Bull.,  i. 

1898. 
Johnston:  Telencephalon  of  selachians.     Jour.  Comp.  Neurol,,  21,  1911. 
Johnston:  Telencephalon  of  ganoids  and  teleosts.     Jour.  Comp.  Neurol,,  21.  1911 
Kappers:  Teleost  and  selachian  brain.     Jour.  Comp.  Neurol.,  16,  1906, 
Kingsbury,     Oblongata  in  fishes.     Jour.  Comp.  Neurol.  7,  1897, 
Locy:  Contribution  to  structure  and  development  of  vertebrate  head.     Jour.  Morph.,  11, 

1895, 

Mayer:  Gehim  der  Knochenfische.     Arch.  Anat.  und  Phys.,  1882. 
Mayser:  Gehim  der  Knochenfische  (Cyprinoids) .     Zeit.  wiss.  Zool.,  36,  i88r. 
Neal:  Segmentation  of  nervous  system  in  Acanthias.     Bull,  Mus.  Comp.  Zool.,  31,  1898. 
Nicholls:  Reissner's  Fibre.     Anat.  Anz.,  40,  1912. 
Sargent:  Reissner's  fibre.     Bull.  Mus,  Comp.  Zool.,  45,  1904. 
Sargent:  Toms  longitudinalis  of  teleost  brain.     Mark  Anniv.  Vol.,  1904. 
Waldschmidt :  Centralnervensystem  \md  Gemchsorgane  von  Polyterus.   Anat.  Anz.,  2, 
1887. 

Worthington:  Brain  and  cranial   nerves  of  Bdellostoma.     Quar.  Jour    Micr.  Sci.,     49, 
1905. 

Amphibia. 

Burckhardt:  Him  und  Gemchsorgan  von  Triton  und  Ichthyophis.     Zeitsch.  wiss.  Zool., 
52,  1891.     • 

Fish:  Central  nervous  system  of  Desmognathus.    Jour.  Morph.,  5,  1895. 
Fischer:  Amphibiorum  nudorum  neurologiae  specimen  primus.     Berlin,  1843. 
Gage:  Brain  of  Diemyctylus  compared  with  Amia  and  Petromyzon.     Wilder  quarter- 
century  book,  1893. 

Griggs:  Early  development  of  central  nervous  system  in  Amblystoma.     Jour.  Morph., 
21,  1910. 

Kingsbury:  Brain  of  Necturus.     Jour.  Comp,  Neurol.,  5,  1895. 

Kingsley  and  Thyng:  Hypophysis  in  Amblystoma.     Tufts  Coll.  Studies,  i,  1904. 

Osbom:  Brain  of  Amphiuma.     Proc.  Acad.  Nat.  Sci.,  Philadelphia,  1883. 

Osbom:  Intemal  stmcture  of  amphibian  bra'n.     Jour.  Morph,,  2,  1888, 

Waldschmidt:  Nervensystem  der  Gymnophionen.     Jena.  Zeitsch,,  20,  18S6. 

Reptilia. 

Gisi:  Gehirn  von  Hatteria  [Sphenodon],     Zool.  Jahrb.,  Abth,  Anat,,  25,  1907. 
Haller:  Bau  des  Wirbeltiergehims.  II,  Emys.     Morph.  Jahrb,,  28,  1900. 
Herrick:  Brain  of  certain  reptiles.     Jour,  Comp.  Neurol.,  i,  1891;  see  also  vol.  3. 
Herrick:  Brain  of  alligator.     Jour.  Cincinnati  Soc}-,  Nat.  Hist.,  12,  i8qo. 
Humphrey:  Brain  of  Chelydra.     Jour,  Comp.  Neurol.,  4,  1894. 
Koppen:  Anatomic  des  Eidechsensgehirn.     Morph.  Arbeiten,  i. 
Rabl-Riickhard:  Centralnervensystem  des  Alligator.     Zeitsch.  wiss.  Zoo!.,  30. 
Rabl-Riickhard:  Gehim  des  Riesenschlange.     Zeitsch.  wiss.  Zool.,  58,  1894. 


368  BIBLIOGRAPHY. 

Birds. 

Bumm:  Grosshirn  der  Vogel.     Zeitsch.  wiss.  Zool.,  38,  1893. 

Kamon:  Entwicklung  des  Gehirns  des  Hiinchens.     Anat.  Hefte,  30,  1906. 

Streeter:  Spinal  cord  of  ostrich.     Am.  Jour.  Anat.,  3,  1903. 

Turner:  Avian  brain.     Jour.  Comp.  Neurol.,  i,  1891. 

Mammals. 

Bechterew:  Leitungsbahnen  im  Gehirn  und  Riickenmark.     Leipzig,  1898. 
Gronberg:  Untersuchungen  an  Gehirn  von  Erinaceus.     Zool.  Jahrb.,  15,  1891. 
Haller:  Bau  des  Gehirn  von  Mus  und  Echidna.     Morph.  Jahrb.,  28,  1900. 
Herrick:  Brain  of  rodents.     Bull.  Denison  Univ.,  6,  1891. 
Landau:  Das  Katzenshirns.     Morph.  Jahrb.,  38,  1908. 
Sab  in:  Atlas  of  medulla  and  mid  brain.     Baltimore,  19 10, 
Smith:  Brain  of  foetal  Omithorhynchus.     Quar.  Jour,  Micr.  Sci.,  39,  1896. 
Smith:  Morphology  of  brain  in  mammals.     Trans.  Linn.  Socy.  London,  Zool.,  8,  1903. 
Symington:  Commissures  in  marsupialia  and  monotremes.     Jour.  Anat.  and  Phys.,  27, 
1892. 

EPIPHYSIAL  STRUCTURES. 

Beard:  Parietal  eye  of  cyclostomes.     Quar.  Jour.  Micr.  Sci.,  29,  1888. 

Dendy:  Devel.  parietal  eye,  etc.,  in  Sphenodon.     Quar,   Jour.  Micr.  Sci.,   42,   1899. 

Dexter:  Development  of  paraphysis  in  fowl.     Am.  Jour.  Anat.,  2,  1902. 

Eycleshymer:  Paraphysis  and  epiphysis  in  Amblystoma.     Anat.  Anz.,  7,  1892. 

Gaupp:  Zirbel,  Parietalorgan  und  Paraphysis.     Ergebnisse,  7,  1897. 

Hill:  Develop,  of  epiphysis  in  Coregonus.     Jour,  Morph.,  5,  1891;  of  teleosts  and  Amia, 

idem,  9,  1894, 
Kingsbury:  Encephalic  evaginations  in  ganoids.     Jour.  Comp.  Neurol.,  .7,  1897, 
Minot:  Morpholog}'  of  pineal  region  based  on  Acanthias,     Am.  Jour,  Anat.,  i,  1901. 
Nowikoff:  Parietalauge  von  Saurien.     Zeit,  wiss.  Zool,,  96,  19 10, 
Reese:   Develop,    of   paraphysis    and   epiphysis   in    alligator,     Smithson.    Misc.    Coll., 

54,   1910- 
Ritter:  Parietal  eye  in  some  lizards.     Bull.  Mus.  Comp.  Zkx)l.,  20,  1891. 
Spencer:  Pineal  eye  in  Lacertilia,     Quar.  Micr.  Sci.,  27,  1886. 
Warren:  Pineal  region  in  Necturus.     Am.  Jour.  Anat.,  5,  1905. 
Warren:  Pineal  region  in  reptiles.     Am,  Jour.  Anat.,  11,  191 1, 

PERIPHERAL  NERVES. 

Allis:  Cranial  muscles  and  nerves  of  Amia.     Jour,  Morph.,  12,  1897. 

AUis:  Cranial  nerves  in  Scomber.     Jour,  Morph.,  18,  1903. 

Beard:  Branchial  sense  organs  and  associated  ganglia  in  ichthyopsida.     Quar.   Jour. 

Micr.  Sci.,  25,  1885. 
Bowers:  Cranial  nerves  of  Spelerpes.     Proc.  Amer.  Acad.,  36,  1901. 
Braus:  Innervation   der   paarigen   Extremitaten   bei   Selachiern    und    Dipnoer.     Jena. 

Zeitsch.,  31,  1898. 
Brook:  Bau  des  sympathet.    Nervensystems  der  Saugetiere.     Jour.   Morph.,   37,    1907; 

38,  1908. 
Brookover:  Olfactory  nerve,  terminalis  nerve  and  preoptic  sympathetic  in  Amia.     Jour. 

Comp.  Neurol,,  20,  1910;  olfact.  and  terminalis  in  Amiurus,     Idem,  21,  1911, 
Carpenter:  Develop,  oculomotor  and  abducens  nerves  and   ciliary  ganglion  in  chick. 

Bull.  Mus.  Comp.  Zool.,  48,  1906.     - 


SENSE  ORGANS.  369 

Coghill:  Cranial  nerves  of  Amblystoma.     Jour.  Comp,  Neurol.,  12,  1902. 

Cole:  Cranial  nerves  of  Chimaera.     Trans.Roy.  Socy.,  Edinburg,  38,  1896. 

Cole:  Cranial  nerves  of  Gadus.     Trans.  Linn.  Socy.  London,  ZooL,  7,  1898. 

Fischer:  Anat.  Abhandl.  iiber  Perrennibranchiaten  und   Derotremen.     Hamburg,  1854. 

See  also  Amphib.  Nudorum,  etc.,  under  Brain. 
Gegenbaur:  Kopfnerv-en  von  Hexanchus.     Jena.  Zeitsch.,  6,  187 1. 

Hammersten:  Innervation  der  Bauchflossen  bei  Teleostiem.     Morph.  Jahrb.,  42,  191 1. 
Herrick:  Cranial  nerves  of  Menidia.     Jour.  Comp.  Neurol.,  9,  1899. 
Herrick:  Cranial  nerves  of  siluroids.     Jour.  Comp.  Neurol.,  11,  1901. 
Herrick:  Criteria  of  homology  in  peripheral  nerv^ous  system.     Jour.  Comp.  Neurol.,  19, 

1909. 
Herrick:  Peripheral  nervous  system  of  bony  fishes.     Bull.  U.  S.  Fish  Commiss.  for  1898. 
Herrick:  Nervus  terminalis  in  frog.     Jour.  Comp.  Neurol.,  19,  1909. 
Huber:  Sympathetic  nervous  system.     Jour.  Comp.  Neurol.,  7,  1897. 
Huber:  Minute  anat.  of  sympathetic  ganglia.     Jour.  Morph.,  16,  1899.  ^ 

Johnston:  Cranial  nerves  of  Petromyzonts.     Jour.  Comp.  Neurol.,  18,  1908. 
Johnston:  Cranial  nerve  components  of  Petromyzon.     Morph.  Jahrb.,  34,  1905. 
Kunz:  Development  of  sympathetic  in  turties.     Am.  Jour.  Anat.,  11,  191 1;  mammals  and 

birds.     Jour.  Comp.  Neurol.,  20,  1910;  of  amphibia,  idem,  21,  1911;  Evolution  symp. 

syst.  in  vertebrates,  idem,  21,  1911. 
Kupffer:  Entwicklimgsgeschichte  des  Kopfes.     Ergebnisse,  1895. 
Kupffer:  Development  of  cranial  ner\'es.     Jour.  Comp.  Neurol.,  i,  1891. 
Landacre:  Cranial  ganglia  in  Amiurus.     Jour,  Comp.  Neurol.,  20,  1910. 
Landacre:  Epibranchial  placodes  of  Lepidosteus  and  their  relation  to  the  cerebral  ganglia. 

Jour.  Comp.  Neurol.,  22,  1912. 
Locy:  New  cranial  nerve  in  selachians.     Mark  Anniv.  Vol.,  1903.     See  also  Anat.  Anz., 

26,  1905. 
Lubosch:  Nervus  accesorius  Willisii.     Arch.  mikr.  Anat.,  54,  1899. 

Mayhoff:  'Monomorphe'  Chiasma  opticum  der  Pleuronectiden.     Zool.  Anz.,  39,  1912. 
McKebben:  Nervous  terminalis  in  Amphibia.     Jour.  Comp.  Neurol.,  21,  1911. 
Neal:  Development  of  ventral  nerves  in  selachii.     Mark  Anniv.  Vol.,  1903. 
Norris:  Cranial  nerves  of  Amphiuma.     Jour.  Comp.  Neurol.,  18,  1908. 
Parker:  Optic  chia§ma  in  teleosts.     Bull.  Mus.  Comp.  Zool.,  40,  1903. 
Pinkus:  Hirnnerven  des  Protopterus.     Morph.  Arbeiten,  4,  1894. 

Prentiss:  Development  of  hypoglossal  ganglion  in  pig.     Jour.  Comp.  Neurol.,  20,  1910. 
Punnett:  Pelvic  plexus  and  nervus  collector  in  Mustelus.     Phil.   Trans.    192,  B,  1900. 
Sheldon:  Nervus  terminalis  in  carp.     Jour.  Comp.  Neurol.,  19,  1909. 
Stannius:  Peripherische    Nervensystem    der    Fische.     Rostock,    1849. 
Streeter:  Development  of  cranial  and  spinal  nerves  in  occipital  region  of  man.     Am.  Jour. 

Anat.,  4,  1904. 
Strong:  Cranial  nerves  of  amphibia.     Jour.  Morph.,  10,  1895. 

SENSE  ORGANS. 

Okajima:  Sinnesorgane  von  Onychodactylus.     Zeit.  wiss.  Zool.,  94,  1909. 
Osawa:  Sinnesorgane  der  Hatteria  [Sphenodon].     Arch.  mikr.  Anat.,  52,  1898. 
Schwalbe:  Lehrbuch  der  Anatomie  der  Sinnesorgane.     Erlangen,  1883. 

Dermal  and  Lateral  Line  Organs. 

Allis:  Lateral  line  system  in  Amia.     Jour.  Morph.,  2,  1889. 
Allis:  Lateral  sensory  canals  of  Mustelus.     Quar.  Jour.  Micr.  Sci.,  45,  1902. 
Allis:  Lateral  canals  of  Polyodon.     Zool.  Jahrb.,  Abth.  Anat.,  17,  1903. 
Ayers  and  Worthington:  Skin  end  organs  of  trigeminal  and  lateralis  nerves  of  Bdellos- 
stoma.     Am.  Jour.  Anat.,  7,  1907. 

24 


370 


BIBLIOGRAPHY. 


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SmelL 

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SENSE  ORGANS.  37 1 

Seydel:    Nasenhohle    und   Jacobson'sche    Organ    der  Amphibien.     Morph.  Jahrb.,   23, 

1895. 
Strong:  Olfactory  organ  and  smell  in  birds.     Jour.  Morph.,  22,  191 1. 
Wilder:    Xasengegend     von     Menopoma     [Cryptobranchus]     und     Amphiuma.     Zool. 

Jahrb.,  Abth.  Anat.,  5,  1892. 
Wilder:  Lateral  nasal  glands  of  Amphiuma.     Jour.  Morph.,  20,  1909. 
Zukerkandl:  Jacobson'sche  Organs.     Ergebnisse,  18,  1910. 
Zukerkandl:  Jacobsonsorgane  und  Riechlappen  der  Amphibien.     Anat.  Hefte,  41,  1910. 

Eyes. 

Bage:     Retina  of  lateral  eyes  of  Sphenodon.     Quar.  Jour.  Mic.  Sci.,  57,  1912. 

Berger:  Sehorgane  der  Fische.     Morph.  Jahrb.,  8,  1882. 

Brauer:  Augen  der  Tiefseefische.     Verhandl.  deutsch.  zool.  Gesellsch.,  1902. 

Carriere:  Sehorgane  der  Thiere.     Mlinchen,  1885. 

Coming:  Anatomie  der  Augenmuskulatur.     Morph.  Jahrb.,  29,  1900, 

Eggeling:  Augenlider  der  Saugetiere.     Jena.  Zeitsch.,  39,  1904. 

Eigenmann:  Eyes  of  blind  vertebrates.     Biol.  Bull.,  2,  1900;  5,  1903. 

Eigenmann:  Eyes  of  Amblyopsidae.     Arch.  Entw.  Mechan.,  7,  1899. 

Eycleshymer:  Development  of  optic  vesicles  in  amphibia.     Jour.  Morph.,  8,  1893. 

Eycleshymer:  Development  of  optic  vesicles  in  Amphibia.  Jour.  Morph.,  8,  1890. 

Jelgersma:  Ursprung  des  Wirbeltierauges,     Morph.  Jahrb.,  35,  1906. 

Lamb:  Development  of  eye  muscles  of  Acanthias.     Am.  Jour.  Anat.,  i,  1901. 

Locy:  Optic  vesicles  of  elasmobranchs  and  tiieir  relations  to  other  structures.     Jour. 

Morph.,  9,  1894. 
Mall:  Histogenesis  of  retina.     Jour.  Morph.,  8,  1893. 
Peters:  Harder'schen  Driise.     Arch.  mikr.  Anat.,  36,  1890. 
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1899. 

Robinson:  Formation  and  structure  of  optic  ner^e  and  its  relation  to  optic  stalk.     Jour. 

Anat.  and  Phys.,  30,  1896. 
Schaeffer:  Develop,  nasolacrimal  passages  in  man.     Am.  Jour.  Anat.,  13,  1912. 
Studnicka:  Sehnerven  der  Wirbeltiere.     Jena.  Zeitsch.,  31,  1897, 
W>ysse:  Histogenesis  of  retina.     Am.  Nat.,  40,  1906. 
Williams:  Migration  of  eye  in  Pseudopleuronecetes.     Bull.  Mus.  Comp.  Zool.,  40,  1902. 

Ears. 

Ayers:  Vertebrate  cephalogenesis  (large  bibliography).     Jour.  Morph.,  6,  1892. 

Ayers:  Relations  of  hair  cells  of  ear.  Jour.  Morph.,  8,  1893. 

Bridge  and  Haddon:  Air  bladder  and  Weberian  ossicles  of  siluroids.     Phil.  Trans.,  184, 
1893. 

Druner:  Anatomie  und  Entwicklung  des  Mittelohres  beim  Menschen  und  Maus.     Anat. 
Anz.,  24,  1904. 

Gaupp:  Schalleitenden  Apparatus  bei  Wirbeltiere.     Ergebnisse,  8,  1898. 

Kingsbury:  Columella  auris  and  nervous  facialis.     Jour.  Comp.  Neurol.,  13,  1903. 

Kingsley:  Ossicula  auditus.     Tufts  College  Studies,  i,  1900. 

Mall:    Development   Eustachian    tube,    middle  ear,  etc.,  of  chick.     Studies  Biol.  Lab. 

Johns  Hopkins,  4,  1887. 
Norris:  Development  of  auditory  vesicle  in  Amblystoma.     Jour.  Morph.,  7,  1892. 
Okajima:  Entwicklung  d.  Gehororganes  von  Hydnobius.     Anat.  Hefte,  45,  1911. 
Parker:    Hearing  and  allied  senses  in  fishes.     Bull.  U.  S.  Fish.  Comm.  for  1902,  1903. 

See  also  Am.  Nat.,  37,  1903. 
Streeter:  Development  of  labyrinth  and  acoustic  and  facial  nerves  in  human  embr}'o. 

Am.  Jour.  Anat.,  6,  1907. 


372  BIBLIOGRAPHY. 

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1898. 
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1890. 

ALIMENTARY  CANAL. 
Teeth. 

Beard:  Teeth  of  marsipobranchs.     Zool.  Jahrb.,  3,  1889, 

Burckhardt:  Gebiss  der  Sauropsiden.     Morph.  Arbeiten,  5,  1895. 

Cope:  Tritubercular  molar  in  human  dentition.     Jour.  Morph.,  2,  1888. 

Harrison:  Development  and  succession  of  teeth  in  Hatteria  [Sphenodon].     Quart.  Jour, 

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Kiikenthal:  Urspring  und  Entwicklung  der  Saugetierzahne.     Jena.  Zeitsch.,  26,  1892. 
Laaser:  Entw.  der  Zahnleiste  der  Selachier.     Anat.  Anz.,  17,  1900. 
Leche:  Entwicklung  des  Zahnsystem  der  Sauger.     Morph.  Jahrb.,  19,  1892, 
Oppel:  Verdauungsapparat.     Ergebnisse,  13,  1903:  14,  1904:  16,  1906. 
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1888. 
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Osborn:  Trituberculy.     Am.  Nat.,  31,  1897. 

Poulton:  Teeth  and  horny  plates  of  Ornithorhynchus.     Quar,  Jour.  Micr.  Sci.,  29,  1888. 
Rose:  Entwicklung  der  Zahne  des  Menschen.     Arch,  mikr.  Anat,,  38,  1891. 
Rose:  Zahnleiste  und  Eischwiele  der  Sauropsiden.     Anat.  Anz.,  7,  1892, 
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Tomes:  Manual  of  Dental  Anatomy.     Philadelphia,  1898. 
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Wilson:  Tooth  development  of  Ornithorhynchus.     Quar.  Jour,  Micr.  Sci.,  51,  1907. 

Mouth  and  Tongue. 

Flint:  Submaxillary  gland.     Am.  Jour,  Anat.,  2,  1903. 
Gegenbaur:  Unterzunge  der  Saugethiere,     Morph.  Jahrb.,  9,  1884. 
Gegenbaur:  Phylogenese  der  Zunge.     Morph.  Jahrb.,  11,  1886;  21,  1894. 
Gegenbaur:  Gaumenfalten  des  Menschen.     Morph,  Jahrb.,  4,  1878. 
Hammar:  Entwicklung  der  Zunge  und  Speicheldriisen.     Anat.  Anz.,  19,  1901. 
Heidrich:  Mund  und  Schlundkopf hohle  der  Vogel  und  ihre  Driisen.     Morph.  Jahrb.,  37, 

1907. 
Kallius:  Entwicklung  der  Zunge.     Anat.  Hefte,  16,  1901;  28,  1905. 
Kallius:  Entwicklung  der  Zunge.     Anat.  Hefte,  41,  1910. 
Maurer:  Blutgefasse  im  Epithel.     Morph,  Jahrb.,  25,  1887, 
Oeder:  Munddriisen  und  Zahnleiste  der  Anuren,     Jena,  Zeitsch,,  41,  1906. 
Pawlowsky:  Giftdriisen  einiger  Scorpaeniden.     Zool.  Jahrb,,  Abt.  Anat.,  31,  191 1. 
Poulton:  Tongue  of  Perameles.     Quar.  Jour.  Micr.  Sci.,  23,  1883, 
Reichel:  Mundhohldriisen  der  Wirbeltiere,     Morph.  Jahrb.,  7,  1882. 
Wiedersheim:  Kopfdriisen  der  Amphibien.     Zeit.  wiss.  Zool.,  28,  1877, 

Thyreoid  Glands,  Etc. 

Erdheim:  Kiemenderivate  bei  Ratte,  Kaninchen  und  Igel,     Anat.  Anz.,  29,  1906. 


ALIMENTARY  CANAL.  373 

Greil:  Kiemendarmderivate  von  Ceratodus.     Anat.  Anz.  Erganz.  Hefte,  29,  1906. 
Ferguson:  Th\Teoid  in  Elasmobranchs.     Am.  Jour.  Anat.,  11,  1911. 
Gudematsch:  Thyreoid    of    Teleosts.     Jour.  Morph.,  21,  1911. 
Hammar:  Elasmobranch   Thymus.     Zool.  Jahrb.,   Abt.   Anat.,   32,    1911. 
Johnstone:  Thymus  in  marsupials.     Jour.  Linn.  Socy.,  London,  Zool.,  26,  1898. 
Kastschenko:  Schicksal    d.    embryon.     Schlundspalten   bei    Saugetieren.      Arch.    mikr. 

Anat.,  30,  1887. 
Marcus:  Schlundspaltgebiet  der  Gymnophionen.     Arch.  mikr.  Anat.,  71,  1908. 
Maurer:  Schilddruse,  Thymus  und  Kiemenreste  der  Amphibien.     Morph.  Jahrb.,  13, 

1887. 
Xorris:  Ventraler  Kiemenreste  and  Corpus  propericardiale  of  the  frog.     Anat.  Anz.,  21, 

1902.    - 
Piatt:  Development  of  Thyroid  and  suprapericardial  bodies  in  Necturus.     Anat.  Anz., 

II,  1896. 
Rabl:  Anlage  der  ultimobranchialen  Korper  bei  Vogel.     Arch.  mikr.  Anat.,  70,  1907. 
SchaflFer:  Schilddruse  von  Myxine.     Anat.  Anz.,  28,  1906. 

Soderlund  imd  Bachman:  Studien  liber  Thymusinvolution.     Arch.  mikr.  Anat.,  73,  1909. 
Stockard:  Development  of  thyreoid  in  Bdellostoma.     Anat.  Anz.,  29,  1906. 
Zuckerkandl:  Entwicklung  der  Schilddruse  und  Thymus  bei  der  Ratte.    Anat.  Hefte,  21, 

1903. 

Digestive  Tract. 

Boas:  Magen  der  Cameliden.     Morph.  Jahrb.,  16,  1890. 

Brachet:  Developpement  du  foie  et  pancreas  de  TAmmocoetes.     Anat.  Anz,,  13,  1897. 

Bensley:  Pancreas  of  guinea  pig.     Am.  Jour.  Anat.,  12,  1911. 

Braun:  Pancreas  bei  Alytes.     Morph.  Jahrb.,  36,  1906. 

Claypole:  Enteron  of  lamprey.     Proc.  Am.  Micros.  Socy.,  1894. 

Choronshitzky:  Entstehimg    der    Milz,    Leber,    GaUenblase,    Bauchspeicheldriise    und 

Pfortadersystem  bei  verschieden.  Wirbeltiere.     Anat.  Hefte,  13,  1900. 
Eggeling:  Dunndarmrelief  und  Emahrung  bei  Knochenfischen.     Jena.  Zeitsch.,  43,  1907. 
Goppert:  Entwicklung  des  Pancreas  bei  Knochenfischen.     Morph.   Jahrb.,   20,   1893. 
Gadow:  Verdauungssystems   der  Vogel.     Jena.  Zeitsch.,    13,    1879. 
Helbling:  Darm  einiger  Selachier.     Anat.  Anz.,  22,  1903. 
Helly:  Pancreasentwicklung  der  Saugetiere.     Arch.  mikr.  Anat.,  67,  1901. 
Howes:  Intestinal  canal  of  Ichthyopsida.     Jour.  Linn.  Socy.  London,  Zool.,  23,  1890. 
Johnston:  Limit  between  ectoderm  and  entoderm  in  mouth  of  amphibia.     Am.  Jour. 

Anat.,  10,  1910. 
Jungklaus:  Magen  der  Cetaceen.     Jena.  Zeitsch.,  32,  1898. 

Kerr:  Development  of  alimentary  tract  in  Lepidosiren.    Quar.  Jour.  Micr.  Sci.,  54,  1910. 
Killian:  Bursa  und  Tonsilla  phar}'ngea.     Morph.  Jahrb.,  14,  1888. 
Kingsbury:  Enteron  of  Necturus.     Proc.  Am.  Micros.,  1894. 
Lewis  and  Thyng:  Intestinal  diverticula  in  embr}-o5  of  pig,  rabbit  and  man.     Am.  Jour. 

Anat.,  7,  1908. 

Mayr:  Entwicklung  des  Pancreas  bei  Selachier.     Anat.  Hefte,  8,  1897. 

Mayer:  Spiraldarm  der  Selachier.     Neap.  Mittheil.,  12,  1897. 

Oppel:  Verdauungsapparat.     Ergebnisse,  7,  1897. 

Osawa:  Eingeweiden  der  Hatteria  [Sphenodon].     Arch.  mikr.  Anat.,  49,  1897. 

Parker:  Spiral  valve  in  Raia.     Trans.  Zool.  Socy.  London,  11,  1880. 

Piper:  Entwicklung  von  Magen,  Duodenum,  Sch'w-immblase,  Leber,  Pancreas  und  ^lilz 

bei  Amia.  Arch.     Anat.  und  Physiol.,  1902. 
Pohlman:  Development  of  cloaca  in  human  embryos.     Am.  Jour.  Anat.,  12,  1911. 
Rex:  Morphologie  der  Saugerleber.     Morph,  Jahrb.,  14,  1888. 


374  BIBLIOGRAPHY. 

Rlickert:  Entwicklung   des   Spiraldarmes   bei   Selachiern.     Arch.  f.    Entwick.  mechan. 

4,  1896. 
Segeratsrale:  Teleostierleber.     Anat.  Hefte,  41,  1910. 

Stieda:  Bau  und  Entwicklung  der  Bursa  Fabricii.     Zeit.  wiss.  Zool.,  34,  1880. 
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Thyng:  Pancreas  in  embryos  of  pig,  rabbit,  cat  and  man.     Am.  Jour.  Anat.,  7,  1908. 
Volker:  Entwicklung  des  Pancreas  bei  den  Amnioten.     Arch.  mikr.  Anat.,   59,    1901. 

RESPIRATORY  ORGANS. 

General. 

Clemenz:  Aussere  Kiemen  der  Wirbeltiere.     Anat.  Hefte,  5,  1904. 

Goppert:  Kehlkopf  der  Amphibien  und  Reptilien.     Morph.  Jahrb.,  22,  1904;   26,  1898; 

28,  1899. 
Gotte:  Ursprung  der  Lunge.     Zool.  Jahrb.,  Anat.  Abth.,  21,  1905. 
Miller:  Structure  of  the  lung.     Jour.  Morph.,  8,  1893. 

Moser:  Entwicklungsgeschichte    der    Wirbeltierlunge.      Arch.    mikr.    Anat.,    60,    1902. 
Oppel:  Athmungsapparat.     Ergebnisse,  13,  1903;  14,  1904;  16,  1906. 
Schmidt:  Kehlhugel  der  Amnioten.     Morph.  Jahrb.,  43,  191 1. 
Spengel:  Schwimmblasen,  Lungen  und  Kiementaschen  der  Wirbeltiere.     Zool.   Jahrb. 

Suppl.,  7,  1904. 

Cyclostomes  and  Fishes. 

Babak:  Darmathmung  der  Cobiten.     Biol.  Centralb.,  27,  1907. 

Beaufort:  Schwimmblase  der  Malacopterygii.     Morph.  Jahrb.,  39,  1909. 

Braus:  Embryonal  Kiemenapparat  von  Heptanchus.     Anat.  Anz.,  29,  1906. 

Bridge  and  Haddon:  Air-bladder  and  Weberian  ossicles  of  Siluridae.     Phil.  Trans.,  184, 

1893. 
Corning:  Wundernetzbildes  in  Schwimmblase  der  Teleostier.     Morph.  Jahrb.,  14,  1888. 
Dahlgren:  Breathing  valves  of  teleosts.     Zool.  Bull.,  2,  1898. 
Dohrn:     Urgeschichte,     u.s.w.      Spritzlochkieme    der      Selachier,    Opercularkieme    d. 

Ganoiden,  Pseudobranchie  der  Teleostier.     Neapel.  Mitth.,  7,  1886. 
Greil:  Homologie  der  Anamnierkiemen.     Anat.  Anz.,  28,  1906. 
Jaeger:  Physiologie  der  Schwimmblase.     Biol.  Centralbl.,  24,  1904. 
Kellicott:  Develop,     vase,   and  respiratory  systems  of  Ceratodus.     Mem.  N.  Y.  Acad. 

Sci.,  2,  1904. 
Mauer:  Pseudobranchien  der  Knochenfisches.     Morph.  Jahrb.,  9,  1883. 
Moroff:  Entwicklung  der  Kiemen  der  Knochenfischen.     Arch.  mikr.  Anat.,  60,  1902. 
Moser:  Entwicklung  der  Schwimmblase.     Arch.  mikr.  Anat.,  63,  1904. 
Muller:  Entwicklung  und  Bedeutung  der  Pseudobranchie  bei  Lepidosteus.     Arch.  mikr. 

Anat.,  49,  1897. 
Nusbaum:  Gasdriise  in  Schwimmblase.     Anat.  Anz.,  31,  1907. 
Rand:  Functions  of  spiracle  in  skate.     Am.  Nat.,  41,  1907.     See  also  Darbyshire,  Jour. 

Linn.  Socy.,  Zool.,  30,  1907. 
Stockard:  Development  of  mouth  and  gills  in  Bdellostoma.  Am.  Jour.  Anat.,  5,  1906. 
Thilo:  Luftsacke  bei  Kugelfische.     Anat.  Anz.,  16,  1899. 
Wiedersheim:    Ein    Kehlkopf    bei    Ganoiden   und    Dipnoern.     Zool.  Jahrb.     Suppl.  7, 

1904- 
Zograff:  Labyrinthine  apparatus  of  labyrinthine  fishes.     Quar.  Jour.  Micr.  Sci.,  28,  1889. 

Amphibia. 

Bruner:  Smooth   facial   muscles  of  anura  and  salamandrina  (respiratory  mechanism). 
Morph.  Jahrb.,  29,  1901. 


CIRCULATION.  375 

Greil:  Anlage  der  Lungen  und  Ultimobranchialkorper.     Anat.  Hefte,  29,  1905. 

Fox:  Tympano-Eustachian  passage  in  toad.     Proc.  Acad.  Nat.  Sci.,  Phila.,  1901 

Martens:  Entwicklung  der  Kehlkopfknorpel  bei  Anuren.     Anat.  Hefte,  9, 1897. 

Ochsner:  Lung  of  Necturus.     Bull.  Univ.  Wisconsin,  ;^;^,  1900. 

Seelyee:  Circulator)^  and  respirator}-  systems  of  Desmognathus.     Proc.    Boston    Socy., 

Nat.  Hist.,  32,  1906. 
Whipple:  Ypsiloid  apparatus  of  Urodeles.     Biol.  Bull.,  10,  1906. 
Wilder:  Phylogenesis  of  larynx.     Anat.  Anz.,  7,  1892. 
Whipple:  Xaso-labial  groove  of  salamanders.     Biol.  Bull.,  11,  1906. 
Wilder:  Amphibian  larynx.     Zool.  Jahrb.  Abth.  Anat.,  9,  1896. 
Wilder:  Lungless  salamanders.  Anat.  Anz.,  9,  1894;   12,  1896. 
Wilder:  PharAngeo-oesophageal  lung  of  Desmognathus.     Am.  Nat.,  35,  1901. 

Sauropsida. 

Cope:  Lungs  of  ophidia.     Proc.  Am.  Phil.  Socy.,  $;i,  1904. 
Gage:  Aquatic  respiration  in  soft-shelled  turtles.     Am.  Nat.,'  20,  1886. 
Hacker:  Unter  Kehlkopf  der  Singvogel.  Anat.  Anz.,  14,  1898. 
Heidrich:  Mund-Schlundhohle  der  Vogel.  Morph.  Jahrb.,  37,  1907. 
Huxley:  Respirator}-  organs  of  Apter}'x.     Proc.  Zool.  Socy.  London,  1882. 
Milani:  Reptilienlungen.     Zool.  Jahrb.  Abth.  Anat.,  8,  1894;  10,  1897. 
Miiller:  Air  sacs  of  pigeon.     Smithsonian  Misc.  Coll.,  50,  1907. 
Sappey:  Recherches  sur  I'apparaeil  respiratoire  des  oiseaux.     Paris,  1847, 
Strasser:  Luftsacke  der  Vogel.     Morph.  Jahrb.,  3,  1877. 

Mammals. 

Bremer:  Lungs  of  opossum.     Am.  Jour.  Anat.,  3,  1904. 

Dubois:  Morphologie  des  Larynx.     Anat.  Anz.,  i,  1886. 

Fox:  Phar\-ngeal  pouches  and  their  derivatives.     Am.  Jour.  Anat.,  8,  1908. 

Goppert:  Herkunft  des  Wrisberg'schen  Knorpels.     Morph.  Jahrb.,  21,  1894. 

His:  Bildungsgeschichte  der  Lungen  bei  mensch.  Embr}-onen.    Arch.  Anat.  und  Phvs., 

1887. 
Justesen:  Entwicklung  und  Verzweigtmg  des  Bronchialbaumes  der  Saugetierlunge.     Arch. 

mikr.  Anat.,  56,  1900. 
Mall:  Branchial  clefts  and  thymus  of  dog.     Johns  Hopkins  Studies  Biol.  Lab.,  4,  1888. 
Shaeffer:  Sinus  maxillaris  in  Man.     Am.  Jour.  Anat.,  10,  1910. 
Schaeffer:  Lateral  walls  of  cavum  nasi  in  man.     Jour.  Morph.,  21,  1910. 
Symington:  The  marsupial  lar}-nx.     Jour.  Anat.  and  Physiol.,  ;^^,  1898;  35,  1899. 

CIRCULATION. 

General. 

AUis:  Pseudobranchial  and  carotid  arteries  in  gnathostomes.   Zool.  Jahrb.,  Abth.  Anat..  27, 

1908. 
Ayers:  Morpholog}-  of  the  carotids.     Bull.  Mus.  Comp.  Zool.,  17,  1889. 
Boas:  Arterienbogen  der  Wirbelthiere.     Morph.  Jahrb.,  13,  1887. 
Broman:  Entwicklung,  'Wanderung'  und  Variation  der  Bauchaortenzweige  bei  Wirbel- 

tieren.     Ergebnisse,  16,  1906. 
Greil:  Anatomic  und  Entwicklung  des  Herzens  und  Truncus  arteriosus  der  Wirbelthiere. 

Morph.  Jahrb.,  31,  1903. 
Greil:  Entwicklung  des  Truncus  arteriosus  der  Anamnier.    Verhandl.     Anat.  Gesellsch., 

17,  1903. 
Grosser:  Kopfvenensystem  der  Wirbeltiere.    Verh.  Anat.  Gesellsch.,  21,  1907. 
Hochstetter:  ^'ergl.  Anat,  und  Entwicklung  des  Venensystem  der  Amphibien  und  Fische, 

Morph.  Jahrb..  13,  1888. 


376  BIBLIOGRAPHY. 

Hochstetter:  Entwicklungsgeschichte  des  Gefasssystem.     Ergebnisse,    i,  1S92. 
Howell:  Life  history  of  the  formed  elements  of  the  blood.     Jour.  Morph.,  4,  1890. 
Lewis:  Development  of  the  vena  cava  inferior.     Am.  Jour.  Anat.,  i,  1902. 
Lewis:  Sinusoids.     Anat,  Anz.,  25,  1904. 

Rose:  Vergl.  Anat.  des  Herzens  der  Wirbeltiere.     Morph.  Jahrb.,  16,  1890. 
Weidenreich:  Die  roten  Blutkorperchen.     Ergebnisse,  13,  1903. 
Weidenreich:  Morphologic  der  Blutzellen.     Anat.  R,ecord,  4,  1910. 
Wright:  Histogenesis  of  blood  platelets.     Jour.  Morph.,  21,  1910. 

Weidenreich:    Blut  und  Blutbildenden  und  -zerstorenden  Organe.     Arch.  mikr.  Anat. 
65-72,  1904-8. 

Fishes. 

Allen:  Blood-vascular  system  of  Loricati.     Proc.  Washington  Acad.  Sci.,  7,  1905. 

Allis:  Pseudobranchial  and  carotid  arteries  in  Polypterus  and  Amiurus.     Anat.  Anz.,  ^;^, 

1908;  in  Esox,  Salmo,  Gadus  and  Amia,  1.  c,  41,  1912. 
Allen:  Subcutaneous  vessels  in  head  of  Polyodon  and  Lepidosteus.     Proc.   ^^'a5hington 

Acad.  Sci.,  9,  1907. 
Carazzi:  Sistema  arteriosa  di  Squalidi.     Anat.  Anz.,  36,  1905. 
Danforth:  Heart  and  arteries  of  Polyodon.     Jour.  Morph.,  23,  1912. 
Hofifmann:  Entwicklung  des  Herzens  und  Blutgefasse  bei  Selachiern.     Morph.  Jahrb., 

19,  1893.     Venensystem,  idem,  20,  1893. 
Holbrook:  Origin  of  endocardium  in  bony  fishes.     Bull.  Mus.  Comp.  Zool.,  25,  1894. 
Jackson:  Vascular  system  of  Bdellostoma.     Jour.  Cincinnati  Socy.  Nat.  Hist.,  20,  1901. 
Allen:  Subcutaneous  vessels  in  tail  of  Lepidosteus.     Am.  Jour.  Anat.,  8,  1908. 
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Acad.  Sci.,  2,  1905. 

Mayer:  Entwicklung  des  Herzens  u.  d.  grossen  Gef^ssstamme  bei  Selachier.     Mittheil. 

zool.  Sta.  Neapel,  7,  1887;  see  also  8,  1888. 
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Socy.  Nat.  Hist.,  29,   1899. 
Parker:  Blood-vessels  of  heart  of  Orthagoriscus.     Anat.  Anz.,  17,  1900. 
Rand:  Posterior  connections  of  lateral  vein  in  skates.     Am.  Nat.,  39,  1905. 
Rex:  Hirnvenen  der  Elasmobranchier.     Morph.  Jahrb.,  17,  1891. 
Senior:  Conus  arteriosus  in  Tarpon  and  Megalops.     Biol.  Bull.,  12,  1907. 
Senior:  Development  of  heart  in  shad.     Am.  Jour.  Anat.,  9,  1909. 
Silvester:  Blood-vascular  system  of  Lopholatilus.     Bull.  Bureau  of  Fisheries,  24,  1904. 
Sobotta:  Entwicklung  des  Blut,  Herzens  und  grossen  Gefassstamme  der  Salmoniden. 

Anat.  Hefte,  19,  1902. 

Amphibia. 

Bethge:  Blutgefasssystem  von  Salamandra,  Triton  und  Spelerpes.     Zeit.  wiss.  Zool.,  6^, 
1898. 

Bruner:  Heart  of  lungless  salamanders.     Jour.  Morph.,  16,  1900. 

Huxley:  Skull  and  heart  of  Menobranchus  [Necturus].     Proc.  Zool.  Socy.  London,  1874. 
Hopkins:  Heart  of  lungless  salamanders.     Am.  Nat.,  30,  1896. 

Marshall  and  Bles:  Development  of  blood-vessels  in  frog.     Studies  Biol.  Lab.  Owens' 
College,  2,  1890. 

Maurer:  Kiemen  und  ihre  Gefasse  bei  Amphibien.     Morph.  Jahrb.,  14,  18S8. 
Miller:  Blood-  and  lymph-vessels  of  lung  of  Necturus.     Am.  Jour.  Anat.,  4,  1905. 
Parker:  Persistence  of  left  postcardinal  vein  in  frog;  homologies  of  veins  in  Dipnoi.     Proc. 

Zool.     Socy.  London,  1889. 
Rabl:  Bildung  des  Herzens  der  Amphibien.     Morph.  Jahrb.,  12,  1887. 
Rex:  Hirnvenen  der  Amphibien.     Morph.  Jahrb.,  19,  1892. 


CIRCULALION.  377 

Romeiser:  Abnormal  venous  system,  in  Necturus.     Am.  Xat.,  39,  1906. 
Santhoff  and  van  \'orhis:  Vascular  system  of  Necturus.     Bull.  Univ.  Wise,  s^,  1900. 
Seelye:  Circulatory  and  respirator}'  systems  of  Desmognathus.     Proc.  Boston  Socy.  Nat. 
Hist.,  32,  1906 

Sauropsida. 

Bruner:  Cephalic  veins  and  sinuses  of  reptiles.     Am.  Jour.  Anat.,  7,  1907. 
Davenport:  Carotids  and  Botall's  duct  of  alligator.     Bull.  Mus.  Comp.  Zool.,  24,  1893. 
Evans:  Earliest  blood-vessels  in  anterior  limbs  of  birds.     Am.  Jour.  Anat.,  9,  1909. 
Grosser  and  Brezina:  Entwicklung  der  Venen  des  Kopfes  und    Raises  der  Reptilian. 

Morph.  Jahrb.,    23,    1895. 
Hochstetter:  Ent%\-icklungsgeschichte  des  Venensystems  der  Amnio  ten.  Reptilian.     Morph. 

Jahrb.,  19,  1892. 
Hochstetter:  Arterien  des  Darmcanals  der  Saurier.     Morph.   Jahrb.,  16,   1898. 
Mackay:  Development  of  branchial  arches  in  birds.     Trans.  Roy.  Socy.   London,    179, 

1888. 
Miller:  Development  of  postcava  in  birds.     Am.  Jour.  Anat.,  2,  1903. 
Stromsten:  Anat.  and  develop,  venous  system  of  Chelonia.    Am.  Jour.  Anat.,  4,  1905. 

Mammals. 

Baddard:  Az}gos  veins  in  mammals.     Proc.  Zool.  Socy.  London,  1907. 
Bom:  Entwicklungsgeschichte  des  Saugetierherzens.     Arch.  mikr.  Anat.,  s^,   1889. 
Davis:  Chief  veins  in  early  pig  embryos.     Am.  Jour.  Anat.,  10,  1910. 
Dexter:  Vitelline  veins  of  cat.     Am.  Jour.  Anat.,  i,   1902. 

Goppert:  Entwicklung  von  Varietaten  im  Arteriensystem  der  weissen  Maus.     Morph. 
Jahrb.,  40,  1909. 

Hochstetter:    Entwicklungsgeschichte    das    Vanensystams    der    Anmiotan.    Mammalia. 

Morph.  Jahrb.,  20,  1893. 
Hochstetter:  Venensystem  dar  Edantaten.     Morph.  Jahrb.,  25,  1897. 
Lewis:  Development  of  vena  cava  inferior.     Am.  Jour.  Anat.,  i,  1902. 
Lewis:  Development  of  veins  in  limbs  of  rabbit.     Am.  Jour.  Anat.,  5,  1905. 
McClure:  Abnormalities  in  postcava  of  cat.     Am.  Nat.,  34,  1900. 
McClure:  Anatomy  and  development  of  venous  system  of  Didelphys.     Am.  Jour.  Anat., 

2,  1903;  5,  1905- 
Minot:  Veins  of  Wolffian  body  of  pig.     Proc.  Boston  Socy.  Nat.  Hist.,  28,  1898. 
Parker  and  Tozier:  Thoracic  derivatives  of  postcardinals  in  swine.     Bull.  Mus.   Comp. 

Zool.,  31,  1898. 
Reagan:  Fifth  aortic  arch  in  mammals.     Am.  Jour.  Anat.,  12,  1912. 
Rose:  Ent^sdcklung  des  Saugetierherzens.     Morph.  Jahrb.,  15,  1890. 
Salzar:  Entwicklung  der  Kopfvanen  des  ^leerschweinnchens.     Morph.  Jahrb.,  23,  1895 
Sichar:  Entwicklung  dar  Kopfarterien  von  Talpa.     Morph.  Jahrb.,  44,  1912 
Tandler:  Anatomie  der  Kopfarterien  bei  Mammalia.     Anat.  Hafta,  18,  .1901. 

Lymphatics. 

Allen:  Lymphatics  of  Scorpaenichthys.     Proc.  Washington  Acad.  Sci.,  8,  1906.    Am.  Jour. 
Anat.,  II,  1911. 

Allen:  Subcutaneous  vassals  in  tail  of  Lapidosteus.     Anat.  Record,  3,  1908. 
Baetjar:  Mesenteric  lymph  sac  in  pig.     Anat.  Record,  2,  1908. 
Budge:  Lymphherzen  bei  Hiihnerembr}'onen.     Arch.  Anat.  u.  Physiol.,  1887. 
Helly:  Hamolymphdriisen.     Ergebnissa,  12,  1902. 

Hopkins:  Lvmphatics  and  enteric  epitheilum  of  Amia.     Wilder  Quarter  Century  Book, 
1893. 

Hoyer    und    Udziela:    Lymphgefasssystem    von    Salamanderlarv^en.     Morph.    Jahrb 
44,  1912. 


378  BIBLIOGRAPHY. 

Huntington:  Anatomy  and  development  of  systematic  hmphatic  vessels  of  cat.     Memoirs 

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Anat.  and  Anat.  Record. 
Killian:  Bursa  und  tonsilla  pharyngea.     Morph.  Jahrb.,  14,  1888. 
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Lewis:  Development  of  lymphatics  in  rabbit.     Am.  Jour.  Anat.,  5,  1905. 
McClure:  Development  of  lymphatics  in  cat.     Anat.  Anz.,  32,  1908. 
Marcus:  Intersegmentale  Lymphherzen  der  Gymnophionen.     Morph.  Jahrb.,  38,   1908. 
Maurer:  Anlage  der  Milz  und  lymphat.  Zellen  bei  Amphibien.     Morph.  Jahrb.,  16,  1890. 
Meyer:  Heemolymph  glands  of  sheep.     Anat.  Record,  2,  1908. 
Miller:  Development  of  jugular  lymph  sac  of  birds.     Am.  Jour.  Anat.,  12,  1912. 
Miiller:  Lymphherzenz  Chelonier.     Abhandl.  Berlin  Acad.,  1839. 

Sabin:  Origin  of  lymphatic  system  in  pig.     Am.  Jour.  Anat.,  i,  1902;  3,  1904;  4,  1905. 
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UROGENITAL  ORGANS. 

GeneraL 

Bardeleben:  Spermatogenese  bei  Menschen.     Jena.  Zeitsch.,  31,  1898. 

Born:  Entwicklung  der  Geschlechtsdriise.     Ergebnisse,  4,  1895. 

Disselhorst:  Harnleiter  der  Wirbeltiere.     Anat.  Hefte,  4,  1894. 

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Arch.  mikr.  Anat.,  33,  1889. 

Cyclostomes  and  Fishes. 

Allen:  Origin  of  sex-cells  of  Amia  and  Lepidosteus.     Jour.  Morph.,  22,  191 1. 
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UROGENITAL  ORGANS.  379 

Balfour  and  Parker:  Lepidosteus.     Phil.  Trans.,  1882, 

Haller:   Phylogenese  des  Nierenorganes  der  Knochenfische.     Jena.  Zeitsch.,  43,  1908. 

Kerr:  Male  geni to-urinary  organs  of   Lepidosiren  and  Protopterus.     Proc.  Zool.  Socy. 

London,  1901. 
Krall:  Mannliche  Beckenflosse  von  Hexanchus.     Morph.  Jahrb.,  37,  1908. 
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1897. 
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Anz.,  2,  1SS8. 
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de  Biol.,  5,  1884. 


DEFINITIONS  OF  SYSTEMATIC  NAMES. 


Acanthias,  genus  of  sharks  including  com- 
mon dogfish. 

Acipenser,  genus  of  ganoids;  sturgeon. 

Aglossa,  tongueless  toads  from  Africa  and 
South  America. 

Aliantoidea,  the  higher  vertebrates  with 
allantois;  reptiles,  birds  and  mammals. 

Amblystoma,  genus  of  tailed  amphibians, 
largely  American. 

Amia,  genus  of  ganoid  fishes  peculiar  to 
America. 

Ammocoetes,  the  larval  stage  of  the  lam- 
preys. 

Amniotes,  division  of  vertebrates  with 
amnion  and  allantois  in  development; 
reptiles,  birds  and  mammal. 

Amphibia,  class  of  vertebrates,  yoimg  with 
gOls,  adults  with  lungs;  frogs,  toads  and 
salamanders. 

Amphioxus,  fish-like  form  without  verte- 
brae, type  of  group  of  Leptocardii. 

Amphipnous,  eel-like  fishes  from  India. 

Amphisbaenans,  legless  lizards. 

Amphiuma,  genus  of  tailed  amphibians 
with  rudimentary  legs  and  gill  slits; 
southern  U.  S. 

Anallantoidea,  vertebrates  without  an 
allantois;  ichthyopsida. 

Anamnia,  vertebrates  without  an  amnion; 
ichthyopsida. 

Anguis,  genus  of  foodess  lizards. 

Anser,  genus  of  birds  including  geese. 

Anthropoids;  sub  order  of  primates  includ- 
ing the  higher  apes  and  man. 

Anura,  order  containing  the  tailless  amphib- 
ians; frogs  and  toads. 

Aquila,  genus  of  birds  including  eagles. 

Archaeopteryx,  a  fossil  bird  with  teeth  and 
a  reptilian  tail 

Archegosaurus,  genus  of  extinct  stego- 
cephal  amphibians 

Arcifera,  group  including  toads  and  tree 
toads. 

Arthrodira,  order  of  extinct  dipnoi  (lung- 
fishes)  some  ver}'-  large, 

Artiodactyla,  ungulate  mammals  with  even 
number  of  toes;  cattle,  sheep,  deer. 

Astroscopus,  genus  of  electric  fishes; 
marine. 

Atelodus,  genus  of  two-toed  rhinoceros. 

Aves,  the  class  of  birds. 

Bdellostoma,  genus  of  myxinoids;  hag 
fishes  of  the  Pacific. 

381 


Belone,  genus  of  fishes;  bony  gars. 
Bombinator,    genus    of    European    toads, 

unke. 
Brady  pus,  genus  of  edentate  sloths. 
Branchiosaurus,    genus    of  extinct    stego- 

cephal  amphibia. 
Bufo,  genus  of  amphibians,  toads. 
Buteo,  genus  of  raptorial  birds,  hawks. 
Butyrinus,  genus  of  herring-like  fishes. 

Caecilians,  a  group  of  legless  tropical  am- 
phibians. 
Caiman,  genus  of  crocodiles. 
Calamoichthys,  genus  of  ganoid  fishes  from 

Africa. 
Callopterus,  genus  of  extinct  ganoid  fishes. 
Camptosaurus,   genus  of  extinct  dinosaur 

reptiles. 
Capitosaunis,  genus  of  extinct  stegocepha- 

lous  amphibia. 
Carcharias,  genus  of  sharks;  sand  shark. 
Carinatae,  birds  with  a  keel  to  the  sternum, 

includes  all  living  birds  except  ostriches. 
Carnivores,  order  of  flesh-eating  mammals; 

cats,  dogs,  bears,  weasels,  seals. 
Castor,  genus  of  rodents,  beaver. 
Ceratodus,   genus   of   dipnoi    (lung-fishes) 

from  Australia. 
Ceratophiys,  genus  of  So.  American  toads. 
Cervus,  genus  of  Ungulates,  common  deer. 
Cestracion,    genus    of    sharks    from     the 

Pacific. 
Cetacea,  order  of  mammals,  whales. 
Chauna,  genus  of  So.  America  crane-like 

birds;  hooded  screamers. 
Chelonia,  order  of  reptile  turtles. 
Chelone,  genus  of  turtles,  greec  turde. 
Chelydra,  genus  of  turtles,  snapping  turtle. 
Chelydrosaurus,    genus    of    extinct    stego- 

cephalous  amphibia. 
Chimaera,    genus    of    peculiar    deep-water 

sharks. 
Chimaeroids,    order    of    shark-like    fishes; 

Holocephali. 
Chiroptera,  order  of  mammal  bats. 
Chlamydoselache,  genus  of  primitive  deep- 
sea  sharks  from  Japan. 
Choloepus,  genus  of  edentates,  sloths. 
Chondrostei,  order  of  ganoid  fishes,  stur 

geon. 
Chn-sophr\-s,  genus  of  fishes;  sea  bream  of 

Europe. 
Chr>'sothrix,    a    genus    of    So.    American 

monkeys. 


382 


DEFINITIONS    OF    SYSTEMATIC    NAMES. 


Cistudo,  genus  of  chelonia;  box  turtles. 
Cladoselache,  genus  of  extinct  sharks. 
Clupeidae,  family  of  fishes  including  herring, 

shad,  ale  wives  and  menhaden. 
Cobitis,  genus  of  fishes;  loaches. 
Coregonus,    genus    of    fresh-water    fishes; 

white  fish. 
Crocodilia,  order  of  reptiles  including  the 

alligator. 
Crotalus,  genus  of  snakes,  rattlesnakes. 
Cryptobranchus,  genus  of  tailed  amphibians 

with  permanent  gill  slits;  hellbender  of 

No.  America. 
Cyclostomes;  class  of  vertebrates  without 

jaws,  including  lampreys  and  hag  fishes. 
Cynognathus,  genus  of  extinct  theromorph 

reptiles. 
Cyprinids,    family    of    freshwater    fishes, 

carp,  minnows. 

Delphinus,  genus  of  whales;  dolphins. 

Derotremes,  tailed  amphibia  with  perma- 
nent gill  slits. 

Desmognathus,  genus  of  salamanders. 

Didelphys,  genus  of  marsupials,  opossums. 

Diemyctylus,  genus  of  small  spotted  sala- 
manders. 

Dinosaurs,  extinct  terrestrial  reptiles,  some 
of  enormous  size. 

Dipnoi,  sub-class  of  fishes  with  gills  and 
lungs,  lung-fishes. 

Discosaurus,  genus  of  stegocephalous 
amphibians. 

Dromatherium,  genus  of  extinct,  primitive 
mammals. 

Echidna,  genus  of  monotremes,  spiny  ant- 
eaters  of  Australia. 

Edentates,  order  of  mammals  including 
sloths,  armadillos,  etc. 

Elasmobranchs,  a  sub-class  of  vertebrates 
including  the  sharks   and    skates. 

Embiotocids,  family  of  fishes  from  the 
Pacific  which  bear  living  young;  surf 
perches. 

Epicrium,  genus  of  caecilians. 

Erinaceus,  genus  of  insectivorous  mammals; 
hedgehogs. 

Er>'thrinus,  genus  of  tropical  fishes. 

Euornithes,  a  name  given  to  all  recent  birds. 

Eupomatus,  fresh-water  sunfish. 

Eurycormus,  genus  of  fossil  ganoid  fishes. 

Firmisternia,  anurous  amphibia  with  the 
halves  of  the  sternum  united  to  each 
other;  frogs. 

Fulica,  genus  of  water  bird;  coots. 

Galeocerdo,    genus    of    selachians;     tiger 

sharks. 
Galeopithecus,  a  flying  mammal  from  Asia 

of  uncertain  position. 
Galeus,  genus  of  sharks;  dogfish. 


Gallus,  genus  of  birds  including  the  com- 
mon fowl. 

Gambusia,  genus  of  fishes;  top-minnow. 

Ganoids,  subclass  of  fishes  intermediate 
between  sharks  and  bony  fishes;  stur- 
geon, garpike,  etc. 

Geococcyx,  a  genus  of  cuckoos. 

Geotrition,  a  genus  of  European  salaman- 
ders. 

Gerrhonotus,  genus  of  lizards. 

Glyptodon,  genus  of  edentates  allied  to 
armadillos. 

Gnathostomes,  vertebrates,  which  have  jaws; 
includes  all  except  cyclostomes. 

Gobiids,  family  of  small  fishes,  mostly 
marine;  gobies. 

Gymnophiona,  order  of  amphibia  without 
tail  or  legs;  tropical;  caecilians. 

Gymnotus,  electric  eel  of  So.  America. 

Halmaturus,  genus  of  kangaroos. 
Hatteria,  another  name  for  Sphenodon. 
Heloderma,  poisonous  lizard  from  Arizona; 

Gila  monster.    * 
Heptanchus,    primitive   shark   with   seven 

gill  slits. 
Hexanchus,   primitive  shark  with  six  gill 

slits. 
Holocephali,    order    of    shark-like    fishes; 

Chimaera. 
Hypogeophis,  genus  of  Caecilians. 
Hyracoidea,  order  of  mammals  including 

Hyrax. 

Ichthyophis,  genus  of  caecilians  from  Ceylon. 
Ichthyopsida,  group  of  vertebrates  which 

have  gills;  fishes,  amphibia. 
Ichthyosaurs,  extinct  aquatic  reptiles. 
Iguana,  genus  of  tropical  American  lizards. 
Insectivores,    order    of    small    mammals; 

moles,  shrews,  etc. 
Inuus,  genus   of   macaques    including  the 

Barbarj'  ape. 

Lacerta,  genus  including  the  common 
lizards  of  Europe. 

Lacertilia,  sub-order  of  reptiles  including 
all  lizards. 

Lagenorhynchus,  a  genus  of  dolphins. 

Lepidosiren,  genus  of  lung  fishes  (dipnoi) 
from  South  America. 

Lepidosteus,  genus  of  ganoid  fishes,  gar- 
pike. 

Lopholatilus,  genus  of  teleosts  from  Gulf 
Stream;  tile  fish. 

Macropus,  genus  of  marsupials;  kangaroos. 
Mammals,  class  of  vertebrates,  with  hair, 

nourishing  the  young  with  milk. 
Manatus,  genus  of  sirenians.  manatees. 
Manis,  genus  of  old-world  edentates;  scaly 

ant-eaters. 


DEFINITIONS    OF    SYSTEMATIC    NAMES. 


383 


Marsupialia,  subclass  of  mammals  with 
pouch  for  young,  opossums,  kangaroos, 
etc. 

Megalops,  genus  of  fishes  including  the  tar 
pon. 
•  Melanerpeton,  genus  of  extinct  stegocephal 
amphibians. 

Monodelphia,  subclass  of  mammals,  in- 
cluding all  except  monotremes  and 
marsupials. 

Monotremata,  subclass  of  mammals  with 
cloaca;  includes  duckbill  and  Echidna 
of  Australia. 

Morones,  genus  of  catfishes. 

Mugil,  genus  of  fishes,  mullets. 

Mustelus,  genus  of  small  sharks;  dogfish. 

Myrmecobius,  genus  of  Australian  mar- 
supials. 

Myxine,  genus  of  cyclostomes;  hag  fishes, 

Myxinoids,     group    of    Cylostomes;    hag 
fishes. 

Necturus,  genus  of  aquatic  amphibians 
with  tail  and  external  gills,  central  U,  S. 

Notidanids,  sub-order  of  sharks  with  more 
than  five  gill  clefts. 

Nototrema,  genus  of  South  American  toads 
with  dorsal  brood  sac. 

Ophidia,  sub-order  of  reptiles;  snakes. 

Opisthocomus,  South  American  bird,  type 
of  a  separate  sub-order. 

Opisthodelphys,  genus  of  tropical  American 
tree- toads. 

Omithorhynchus,  genus  of  monotremes; 
duckbill  of  Australia. 

Ostariophysi,  bony  fishes  with  Weberian 
apparatus. 

Ostracoderms,  a  group  of  extinct  verte- 
brates of  very  uncertain  position. 

Palaeohatteria,  a  fossil  reptile  allied  to 
Sphenodon. 

Palaeospondylus,  a  problematical  fossil 
vertebrate  from  Scotland. 

Perennibranchs,  tailed  amphibia  which 
retain  the  gills  through  life. 

Perissodactyls,  sub  order  of  mammals  with 
odd  number  of  toes;  horses,  rhinoceros, 
tapirs. 

Petrobates,  genus  of  extinct  theromorph 
reptiles. 

Petromyzonts,  subclass  of  cyclostomes, 
lampreys. 

Phoca,  genus  of  carnivores  including  com- 
mon seals. 

Physoclisti,  fishes  in  which  the  air-bladder 
is  closed. 

Physostomi,  group  of  fishes  in  which  the  air- 
bladder  has  a  duct;  mostly  fresh  water. 

Pipa,  tongueless  toad  from  South  America. 

Pisces,  the  class  of  fishes. 


Placentalia,  all  mammals  (except  marsupials 
and  monotremes)  in  which  a  placenta 
occurs. 

Placodus,  genus  of  extinct  theriomorph 
reptiles 

Plesiosaurs,  order  of  extinct,  long-necked 
swimming  reptiles 

Polyodon,  genus  of  ganoid  fishes,  paddle 
fish. 

Polypterus,  genus  of  ganoids  from  Africa. 

Porichthys,  genus  of  fishes  from  Pacific; 
midshipman. 

Primates,  highest  order  of  mammals, 
including  monkeys,  apes  and  man. 

Pristiurus,  genus  of  European  dogfish. 

Proboscidea.  order  of  mammals,  including 
elephants. 

Procolophon,  genus  of  extinct  theromorph 
reptiles. 

Proteus,  genus  of  tailed  amphibians  from 
caves  of  Austria,  allied  to  Necturus. 

Protopterus,  genus  of  dipnoi  from  Africa. 

Psittacus,  genus  of  parrots. 

Pterodactyls,  extinct  flying  reptiles. ' 

Pterosaurs,  extinct  flying  reptiles,  ptero- 
dactyls. 

Pythonomorphs,  a  group  of  extinct  swim- 
ming reptiles. 

Raia,  genus  of  elasmobranchs,  including 
the  skates. 

Rana,  genus  of  amphibia,  frogs. 

Ratitae,  birds  without  keel  to  sternum, 
ostriches. 

Rhea,  three-toed  South  American  ostrich. 

Rhynchobatus,  genus  of  tropical  skates. 

Rhynchocephalia,  order  of  lizard-like  rep- 
tiles; Sphenodon  of  New  Zealand  only 
living  species. 

Rodentia,  order  of  mammals  with  gnawing 
teeth,  rats,  rabbits,  beaver. 

Ruminants,  group  of  ungulate  mammals 
which  chew  the  cud. 

Salamandra,  genus  of  tailed  amphibia  from 

Europe. 
Salamandrina,  order  of  tailed  amphibians 

without  gills. 
Salmonids,  family  of  fishes  including  trout 

and  salmon. 
Sauropsida,  class  of  vertebrates  including 

reptiles  and  birds. 
Sceleporus,    genus    of    lizards    of    eastern 

United  States. 
Scomber,  genus  of  fishes;  mackerel. 
Scorpsenichthys,  genus  of  sculpins. 
Selachii,  order  of  elasmobranchs;  sharks. 
Serranidae,    family    of    marine,    perch-like 

fishes. 
Siluroids,    order   of   fishes  containing    the 

cat-fishes. 
Siren,  genus  of  tailed  amphibian  from  U.S. 

with  external  gills. 


3^4 


DEFINITIONS    OF    SYSTEMATIC    NAMES. 


Sirenia,      order     of      marine      mammals; 

manatees  and  dugongs 
Sirenoidea;  order  of  lung-fishes,  containing 

the  living  species. 
Spalacotherium,  genus  of  extinct  mammals. 
Sphenodon,    genus    of    lizard-like    reptiles 

from    New    Zealand;    order    Rhyncho- 

cephalia, 
Squamata,  order  of  reptiles  including  snakes 

and  lizards. 
Stegocephala,  order  of  extinct  amphibians. 
Stegosaurs,     family    of     extinct    dinosaur 

reptiles,  some  very  large. 
Stenops,  genus  of  lemurs. 
Stenostomus,  genus  of  fishes;  scup. 

Teleostomes,  fishes  with  true  jaw,  includes 

ganoids  and  teleosts. 
Teleosts,  order  of  fishes  with  bony  skeleton, 

including  all  common  fishes. 
Testudo,  genus  of  land  turtles. 
Testudinata,  turtles,  same  as  a  Chelonia. 
Tetrapoda,    term    to    include    amphibia, 

reptiles,    birds,    and    mammals,    which 

have  feet  in  place  of  fins. 
Theromorpha,  extinct  reptiles  forming  the 

lowest  order  of  the  class. 
Tinnunculus,    genus    of    hawk-like   birds; 

kestrel. 


Torpedo,'  genus  of  skates  with  remarkable 

electric  powers. 
Trionyx,  genus  of  fresh-water  turtles. 
Triton,  genus  of  tailed  amphibian,  aquatic, 

European. 
Tropidonotus,  genus  of  snakes,   including 

our  water  snake. 
Trygon,  genus  of  skates,  string-rays. 
Typhlopidae,    family    of    peculiar    tropical 

serpents. 

Ungulates,  order  of  mammals  which  walk 
on  the  tips  of  the  toes;  horse,  cattle,  deer, 
antelope,  etc. 

Urodeles,  order  of  tailed  amphibia. 

Varanus,  genils  of  lizards  from  Africa. 

Xenarthra,  sub-order  of  American  edentates, 

ant-eaters  and  armadillos. 
Xenopus,   genus  of  tongueless  toads  from 

Africa. 

Zeuglodon    a    genus     of     extinct     whales 

(Cetacea) . 
Ziphius,  genus  of  toothed  whales. 


INDEX. 


Abdominal  aorta,  284 

pores,  124,  322 

ribs,  41 

sternum,  57 

vein,  289 

vertebras,  49 
Abducens  nerve,  170 
Abomasum,  227 
Accessor}'  nerve,  177 
Acetabular  bone,  112,  113 
x\cetabulum,  104,  109 
Acinous  glands,  18 
Acrodont,  88,  213 

dentition,  213 
Acromion  process,  109 
Actinotrichia,  103 
Activators,  264,  353 
Acustico-Iateralis  nerves,  167 
Acustic  nerve,  174 
Adductor  muscles,  132 
Adenoid  tissue,  307 
Adipose  tissue,  22 
Adrenalin,  353 
Adrenal  organs,  352 
Advehent  vein,  291 
.^githognathous,  97 
Afferent  branchial  artery,  274 

duct  of  gills,  239 

nerve  root,  161 
Air-bladder,  247 

ducts,  251 

sacs,  261 
Ala  orbitalis,  98 

temporalis,  61,  98 
Alimentarv'  canal,  205 
Alisphenoid,  67 

cartilage,  61 
Allantoic  arteries,  278,  285,  293 

bladder,  318 

veins,  350 
Allan tois,  264,  278,  350 
Alveolar  ducts,  256 
Alveoli  of  jaws,  213 

of  lung,  256 
Amnion,  350 
Amniotes,  copulatory  organs,  344 

development  of  heart.  271 
Amniotic  cavity,  ^50 
Amphibia,  brain,  155 

circulation,  295 

dermal  skeleton,  41 

excretory  organs,  327 

gills,  242 


Amphibia,  girdles,  106,  110 

glands,  29 

intestine,  229 

larjTix,  251 

lateral  line  organs,  180 

limbs,  118 

lungless,  258 

lungs,  257 

reproductive  organs,  ^t,^ 

skin,  29 

skuU,  82 

teeth,  212 

thymus,  246 

thyreoid,  247 

tongue,  217 

vertebral  column,  51 
Amphicoelous,  46 
Amphiplatyan,  46 
Amphiohinal,  191  • 
Amphistviic,  73 
Ampullae  of  ear,  184 

of  Lorenzini,  182 

of  Savi,  182 
Amylopsin,  234 
Anchylosis,  38 
Angvdare,  71 
Anlage,  vi. 
Antibrachium,  116 
Anterior  abdominal  vein,  289 

cardinal  vein,  279 

cephalic  duct,  303 

chamber  of  eye,  203 

comua,  139 

process,  74 

vena  cava,  300 
Anthers,  loi 

Antrum  of  Highmore,  197 
Aorta,  273,  284 
Aortic  arches,  273 

arches,  modifications  of,  282 
Aponeurosis,  129 
Appendages,  102,  114 
Appendicular  skeleton,  102 
Apteria,  32 
Aqueduct,  143 
Aqueous  humor,  203 
Arachnoid  membrane,  152 
Arbor  vitae,  161 
Arcades,  11 
Archenteron,  9 
Archiccele,  8 
Archinepteric  duct,  312 
Archipter}gium,  115 


385 


SS6 


INDEX. 


Areolar  tissue,  22 
Argential  layer  of  eye,  202 
Arterial  ring,  287 
Arteries,  266,  284 

afferent  branchial,  274 

allantoic,  278,  285,  293,  350 

axillary,  288 

basilar,  287 

brachial,  288 

branchial,  274 

carotid,  275 

caudal,  276 

central  retinal,  201 

ciliary,  202 

cceliac,  284 

common  carotid,  282 

coronary,  273 

cutaneus,  289 

development  of,  273 

efferent  branchial,  274 

epigastric,  288 

external  iliac,  288 

femoral,  288 

gastric,  284 

genital,  286 

hepatic,  284 

hyaloid,  201 

hypogastric,  276,  285 

iliac,  288 

intercostal,  275,  286 

ischiadic,  288 

lumbar,  286 

mandibular,  271 

mesenteric,  284 

nephridial,  275,  285 

omphalomesaraic,  276 

omphalomesenteric,  276 

ovarian,  286 

peroneal,  288 

popliteal,  288 

pulmonary,  283 

radial,  288 

renal,  286,  300 

sacral,  286 

sciatic,  288 

somatic,  284 

spermatic,  286 

spinal,  287 

splenic,  284 

subclavian,  288 

tibial,  288 

ulnar,  288 

umbilical,  285 

vertebral,  287 

vesical,  285 

visceral,  284 

vitelline,  293 
Articular  bone,  71 
Articular  process,  46 
Articulare,  74 
Articulations,  38 
Arytenoid  cartilage,  251 
Ascending  aorta,  284 


Ascending  process,  82 

tracts,  140 
Asterospondylous,  51 
Astragalus,  117 
Atlas,  49 

Atrial  chamber  of  gills,  240 
Atrioventricular  canal,  272 
Atrium  of  heart,  272 

lungs,  258 

of  nose,  194 
Auditory,  bulla,  100 

meatus,  187 

nerves,  174 

organs,  182 

vesicle,  183 
Auricle  of  heart.  272 
Aiu"icularis  superficialis  nerve,  171 
Autostylic,  73 
Axial  skeleton,  43 
Axillary  artery,  288 

vein,  290 
Axis,  49 
Axon,  19,139 
Azygos  appendages,  103 
Azygos  vein,  302 

Baleen,  216 
Barbs,  31 
Barbules,  31 
Basalia,  103 
Basibranchial,  65 
Basilar  artery,  287 

plate,  60 
Basioccipital,  67 
Basisphenoid,  67 
Basitemporal  plate,  96 
Bicuspids,  213 
Bidder's  organ,  347 
Bile,  231 

duct,  233 
Birds,  see  also  Amniotes,  Sauropsida. 

air- sacs,  261 

brain,  158 

circulation,  300 

gill  pouches,  244 

girdles  of,  108,  113 

intestine,  230 

limbs,  119 

lungs,  259 

scales,  31 

skin,  30 

skull,  95 

stomach,  225 

thymus,  246 

thyreoid,  247 

tongue  of,  218 

vertebral  column  of,  52 
Biserial  fins,  115 
Bladder,  air,  247 

allantoic,  318 

smm,  247 

urinary,  318 
Blastomeres,  8 


INDEX. 


387 


Blastopore,  9 

Blastula,  8 

Blood,  24,  265 

Blood  circulation,  embrj'onic,  268 

phylogeny  of,  267 

primitive,  268 
Blood  corpuscles,  265 

-lymph  glands,  307 

plasma,  265 

plates,  266 

vascular  system,  266 

vessels,  structure  of,  267 
Body  ca\^t}%  120 
Bone,  23 

development  of,  43 

of  ear,  73 
Botall's  duct,  283 
Bowman's  capsule,  309,  314 

glands,  197 
Brachial  artery,  288 

plexus,  163 

vein,  290 
Brachium,  116 
Brain,  140 

flexures  of,  143 

sand,  160 

ventricles  of,  143 
Branchiae,  236 
Branchial  arteries,  274 

arches,  63 

clefts,  236 

vein,  274 
Branchiomerism,  237 
Branchiostegal  membrane,  77,  240 

rays,  77,  240 
Breathing  valves  of  teleosts,  241 
Breast  bone,  56 
Broad  ligament,  337 
Bronchi,  250,  256 
Bronchioles,  256 
Bronchus  of  lampreys,  238 
Buccal  glands,  221 
Buccalis  nerve,  172 
Bulbus  arteriosus,  273 

oculi,  203 

olfactorius,  142,  167 
Bunodont,  214 
Bursa  Entiana,  227 

inguinalis,  338 

omen  talis,  122 

Caeca,  intestinal,  228 

pyloric,  227 
Calcaneus,  117 
Calcareous  glands,  183 
Calcified  cartilage,  43 
Campanula  HaUeri,  204 
Canaliculi,  23 
Canines,  213 
Capillaries,  267 
Capitatum,  117 
Capitular  head  of  rib,  54 
Capsules,  sense,  60 


Carapace,  41 
Cardiac  glands,  224 

plexus,  163 
Cardinal  veins,  279 
Carotid  arteries,  275 
Carotid  glands,  246,  297 
Carpale,  117 
Carpus,  116 
Cartilage,  22 

bones,  43,  66 

calcified,  43 

lingual,  75 

rostral,  76 
Cauda  equina,  140 
Caudal  artery,  276 

vein,  276 

vertebrae,  49 
Cavum  tympani,  187 
Cement,  211 

Central  canal  of  nervous  system,  138 
Centrale,  117 

Central  nervous  system,  11,  137 
Centrum,  45 
Cephalic  vein,  290 
Ceratobranchial,  65 
Cerebellar  hemispheres,  161 
Cerebellum,  142,  145 
Cerebral  hemispheres,  141 
Cerebrospinal  fluid,  152 
Cerebrum,  148 
Cervical  plexus,  163 

sinus,  244 

vertebrae,  49 
Chain  ganglia,  163 
Chambers  of  eye,  203 
Chiasma,  169 
Chiropterygia,  114 
Choanae,  80,  193 
Choledochar  duct,  233 
Chondrin,  23 
Chondrocranium,  60 
Chorda  tympani,  173 
Chordae  tendiniae,  272 
Chordata,  2 
Chorioid  coat,  202 

fissure,  199 

gland,  202 

plexus,  144,  147 
Chorion,  351 
Chromaffine  cells,  352 
Chroma phile  cells,  352 
Chromatophores,  26 
Chyle,  304 
Chyle  ducts,  304 
Cilia,  205 

Ciliary-  arteries,  202 
Ciliary  ganglion,  165,  170 

muscles,  202 

process,  202 
Ciliated  epithelium,  18 
Circle  of  Willis,  287 
Circulation,  allantoic,  278 

foetal,  293 


388 


INDEX. 


Circulation,  hepatic- portai,  277 

portal,  277 

pulmonary,  282 

renal-portal,  280 

respiratory,  282 

systemic,  282 
Circulatory  organs,  264 
Cistern  of  chyle,  303 
Claspers,  27,  116,  343 
Clavicles,  106 
Claws,  27 
Cleft  palate,  193 
Cleithrum,  106 
Cloaca,  228 
Coccyx,  52 
Cochlea,  186 
Cochlear  nerve,  174,  186 
Coeliac  artery,  284 

axis,  285 
Coelom,  10,  14,  120 
Collecting  tubule,  309 
Collector  nerves,  163 
Colon,  228 
Columella  auris,  74 
Columnae  carnea,  272 
Columnar  epithelium,  17 
Columns  of  cord,  139 
Commissura  mollis,  146 
Common  carotid  artery,  282 

iliac  vein,  289 
Concha  of  ear,  188 

of  nose,  194 
Cones  of  eye,  199 
Conjunctiva,  203 
Connective  tissues,  21 
Contour  feathers,  3 1 
Conus  arteriosus,  272 
Convoluted  tubule,  309 
Convolutions  of  brain,  149,  160 
Copulae,  63 

Copulatory  organs,  342 
Coraco-arcual  muscles,  133 
Coracoid  bone,  107 

process,  108 

region,  105 
Corium,  25 
Cornea,  202 
Cornua  of  cord,  139 

trabeculae,  61 
Cornua  radiata,  160 
Coronary  arteries,  273 
Coronoid  bone,  71 
Corpora  adiposa,  307 

bigemina,  142 

quadrigemina,  142,  160 
Corpus  albicans,  151 

callosum,  150 

luteum,  320 

restiforme,  150 

striatum,  141 
Corpusculum  bulboideum,  179 
Cortex  of  cerebrum,  149 
Corti's  organ,  186 


Cotyloid  bone,  113 
Cowper's  glands,  342 
Cranial  bones,  table  of,  72 

nerves,  165 
Cranio-quadrate  process,  82 
Cranium,  60 
Cremaster  muscle,  338 
Cribriform  plate,  67,  100 
Cricoid  cartilage,  251 
Cristae  acusticae,  185 
Crista  galli,  100 
Crop,  223 
Crura  cerebri,  151 
Crus,  116 
Ctenoid  scales,  40 
Cubical  epithelium,  17 
Cuboides,  117 
Cuneiform,  117 
Cutaneus  artery,  289 

magnus  vein,  290 
Cutis,  25 

Cuverian  ducts,  271,  278 
Cycloid  scales,  40 
Cyclospondylous,  51 
Cyclostomes,  brain,  152 

circulation,  294 

ear,  185 

excretory  organs,  321 

eyes,  204 

gills,  238 

intestine,  228 

lateral  line  organs,  180 

mouth,  208 

nasal  organs,  190 

reproductive  organs,  33 1 

skull,  75 

teeth,  215 

thymus,  245 

thyreoid,  246 

tongue,  217 

vertebral  column,  51 
Cylindrical  corpusle,  179 
Cystic  duct,  234 

Decussation  of  fibres,  150 
Deiter's  cells,  186 
Demibranch,  237 
Dendrites,  19,  139 
Dens,  50 
Dental  formula,  2 14 

papilla,  209 

ridge,  210 

shelf,  210 
Dentary  bone,  71 
Dentinal  canals,  24 
Dentine,  24,  209 
Dentitions,  211 
Depressor  mandibulae,  133 

muscles,  131 
Derma,  25 
Dermal  muscles,  134 

skeleton,  38,  39 
Dermarticulare,  71 


INDEX. 


389 


Descending  aorta,  284 

tracts,  40 
Desmognathous,  97 
Deutoplasm,  8 
Diaphragm,  123,  135 
Diapophysis,  46 
Diarthrosis,  38 
Diastole,  272 
Diencephalon,  142 
Digastric  muscle,  133 
Digestive  tract,  12,  205 
Digitigrade,  120 
Digits,  116 
Dilator  pupillae,  202 
Diphycercal,  50 
Diphyodont  dentition,  211 
Dipnoi,  brain,  154 

circulation,  292,  295 

excretory  organs,  327 

lungs,  257 

reproductive  organs,  333 

skull  of,  80 
Discus  proligeru§,  320 
Dorsal  aorta,  275 

fissure  of  cord,  139 

nerve  root,  161 

vertebrae,  49 
Down  feathers,  31 
Dromaeognathous,  97 
Ductless  glands,  18 
Ductus  arantii,  277 

arteriosus,  283 

Botalii,  283 

Cuverii,  271,  278 

venosus,  277 
Dumb-bell  bone,  69,  loi 
Duodenum,  227 
Dura  spinalis,  152 

Ear,  182 

bones,  73 

external,  187 

fimctions  of,  188 

inner,  183 

middle,  187 

stones,  186 
Ect-ental  line,  9 
Ectethmoid,  67 
Ectobronchus,  259 
Ectochondrostosis,  43 
Ectoderm,  9 
Ectopterygoid,  80,  88 
Ectoturbinals,  196 
Efferent  branchial  artery,  274 

duct  of  gills,  238 

nerve  root,  161 
Egg,  segmentation  of,  8 

teeth,  216 
Ejaculator>'  duct,  341 
Elasmobranchs,  brain,  153 

copulator}'  organs,  343 

excretor}'  organs,  326 

gills,  239 


Elasmobranchs,  girdles  of,  105 

intestine,  228 

reproductive  organs,  331 

skull,  76 
Elastica  externa,  45 

interna,  44 
Elastic  tissue,  22 
Electrical  organs,  135 
Electric  plates,  135 
Electroplax,  136 
Embolomerous,  48 
Embryology,  i,  6 
Embryonic  tissue,  22 
Eminentia  medialis,  145 
Enamel,  40 

organ,  40,  209 
Endocardium,  269 
Endolymph,  185 

duct,  183 

sac,  183 
End  organs,  178 
Endorhachis,  151 
Endoskeleton,  38,  42 
Ensiform  process,  56 
Entepicondylar  foramen,  120 
Enteroccele,  10 
Enteropneusta,  2 
Entobronchus,  259 
Entochondrostosis,  43 
Entoderm,  9 
Entoglossal,  80,  97,  218 
Entoplastron,  42,  159 
Entopterygoid,  80 
Entoturbinals,  196 
Entovarial  canal,  326 
Envelopes  of  nervous  system,  15] 
Eparterial  bronchi,  262 
Epaxial  muscles,  127 
Ependyma,  139 
Epibranchial  cartilage,  65 
Epibranchial  ganglia,  176 

muscles,  133 
Epicardium,  124,  269 
Epicoele,  143 
Epicoracoid,  107 
Epidermis,  25 
Epididymis,  322 
Epigastric  artery,  288 

vein,  289 
Epiglottis,  252,  253 
Epimerals,  54 
Epimere,  13 
Epineurals,  54 
Epiotic,  67,  69 
Epipharyngeal  bones,  80 
Epiphyses,  43 
Epiphysial  structures,  146 
Epiphysis,  147 
Epiplastron,  42,  108 
Epipleurals,  54 
Epipter\-goid,  82 
Epipubis,  III 
Episternalia   109 


390 


INDEX. 


Episternum,  59 
Epistropheus,  49 
Epithelial  bodies,  246 

pigmented,  of  eye,  201 
Epithelium,  17 
Epitrichium,  25 
Erectile  tissue,  345 
Erythrocytes,  265 
Essence  of  pearl,  29 
Ethmoidalia,  67 
Ethmoid  bone,  68 

plate,  61 
Ethmopalatine  ligament,  77 
Ethmo-turbinals,  195 
Eustachian  tube,  187,  237 
Excitatory  cells,  164 
Excretory  organs,  307 

development  of,  310 
Exoccipital,  67 
Extensor  muscles,  132 
External  carotid  artery,  275 

ear,  187 

gills,  development  of,  242 

iliac  artery,  288 
Extrabranchial  cartilages,  65 

chamber,  240 
Extrinsic  muscles,  131 
Eyelashes,  205 
Eyelids,  203 
Eye  muscles,  128,  203 
Eye- muscle  nerves,  170 
Eye,  parietal,  147 
Eyes,  198 

Fabellae,  118 
Facialis  nerve,  172   • 
Falciform  process,  204 
Fallopian  tube,  338 
False  amnion,  350 

rib,  55 
Falx  cerebri,  152 
Fascia,  128 
Fasciculi,  128 
Fasciculus  communis,  150 
Fat,  22 

bodies,  307 
Fauces,  isthmus  of,  247 
Feather  tracts,  32 
Feathers,  31 
Femoral  artery,  288 

pores,  30 

vein,  290 
Fenestra  hypophyseos,  61 

ovale,  73,  186 

rotunda,  186 

tympani,  186 

vestibuli,  73,  186 
Fibrous  tissue,  22 
Fibula,  116 
Fibulare,  117 
Fifth  ventricle,  151 
Filoplumes,  31 
Filum  terminale,  140 


Fins,  102 

anal,  103 

biserial,  115 

caudal,  103 

dorsal,  103 

paired,  114 

uniserial,  115 
Fishes,  circulation,  294 

eyes,  204 

fins,  115 

gills,  238 

girdles,  no 

glands,  27 

intestine,  229 

lateral  line  organs,  180 

scales,  40 

skin,  27 

skull,  77 

tails  of,  50 

teeth,  212 

thymus,  245 

thyreoid,  247 

tongue,  217 

vertebral  column,  51 
Fissures  of  brain,  149,  159 
Flexor  muscles,  132 
Flocculi,  145 
Flexures  of  brain,  143 
Floor  plate,  138 
Foetal  circulation,  293 

envelopes,  348 
Folian  process,  74 
Fontanelles,  61 
Forebrain,  140 
Foramen  caecum,  219 

epiploicum,  122 

incisorum,  loi 

interventriculares,  143 

lacerum  anterior,  67 

magnum,  67 

of  Monro,  143 

of  Panniza,  282 

of  Winslow,  122 
Fornix,  151 
Fossa  hypophyseos,  61 

rhomboidea,  144 
Fossae  of  skull,  71 
Fovea  centralis,  200 
Free  appendages,  114 

nerve  terminations,  178 
Frontal  bones,  68 

lobes,  159 

organs,  147 
Fundus  glands,  224 
Furcula,  108 

Gall.  231 

bladder,  233 

capillaries,  233 
Ganglia  of  dorsal  roots,  161 
Ganglion,  20 

cell,  19 

of  retina,  200 


i 


INDEX. 


391 


Ganoids,  excretory  organs,  327 

reproductive  ducts,  322 

scales,  40 

skull  of,  78 
Ganoin,  40 

Gasserian  ganglion,  171 
Gastralia,  41 
Gastric  artery,  284 
Gastrula,  9 
Gastrulation,  9 
Geniculate  ganglion,  172 
General  cutaneus  nerves,  167 
Geniohyoid  muscle,  130 
Genital  artery,  286 

prominence,  344 
Geological  distribution,  7 
Germ  layers,  11 
Gill  arches,  63 

basket,  75 

clefts,  236 

cover,  77,  240 

pouches,  239 

remnants,  246 
Gills,  236 
Girdles,  104 
Gizzard,  225 
Gladiolus,  56 
Glands,  18 

of  amphibia,  29 

of  birds,  30 

buccal,  221 

cardiac,  224 

carotid,  297 

chorioid,  202 

Cowper's,  342 

excretory,  307 

of  fishes,  27 

fundus,  224 

Harder' s,  204 

hibernating,  307 

intermaxillary,  220 

intemasal,  220 

labial,  22 

lacrimal,  204 

lingual,  221 

of  mammals,  35 

mammary,  36 

meiobomian,  205 

milk,  36 

molar,  221 

oral,  220 

orbital,  221 

palatal,  220  221 

parotid,  221 

prostate,  342 

poison,  27,  221 

pyloric,  224 

rectal,  228 

of  reptiles,  30 

retrolingual,  221 

sexual,  308 

sublingual,  221 

submaxillary,  221 


Glands,  tarsal,  205 

tear,  204 

uropygial,  30 
Glandula  membrana  nictitans,  204 
Glandular  epithelium,  18 
Glaserian  fissure,  74 
Glenoid  fossa,  100,  104 
Glia,  139 

cells,  19,  20 
Glomerulus,  309 

Glomeruli  of  olfactory  nerve,  167 
Glomus,  312 

Glossopharyngeal  nerve,  175 
Glottis,  251 
Gluteus  muscle,  132 
Gonads,  308,  319 
Goniale,  71 
Gonotomes,  319 
Graafian  follicle,  320 
Grandry's  corpuscle,  179 
Gray  matter,  20 

of  cord,  139 
Great  omentum,  122 
Guanin,  29 
Gubemaculum,  338 
Gular  bones,  79 
Gyri,  149,  160 

Habenular  ganglion,  146 
Haemopophysial  ribs,  53 
Haemal  spine,  46 
Haemapophysis,  46 
Hair,  33 
Hair  cells,  186 
Hallux,  117 
Hamatum,  117 
Harder's  glands,  204 
Hare-lip,  193 
Haversian  canals,  23 
Head  cavities,  128 

kidney,  310 

rib,  81 
Heart,  281 

branchial,  281 

development,  269 

division  of,  281 

muscles,  125 

portal,  294 

structvu-e,  269 

venous,  281 
Hemiazygos  vein,  302 
Hemipenes,  344 
Hemispheres,  cerebellar,  161 
Hemispheres,  cerebral,  141 
Henle's  loop,  309 
Hepar,  231 
Hepatic  artery,  284 

duct,  233 

-portal  system,  277 

veins,  277 
Herbst's  corpuscle,  179 
Hermaphroditism,  346 
Heterocercal,  50 


392 


INDEX. 


Hibernating  glands,  307 
Highmore,  antrum  of,  197 
Hilum  of  kidney,  330 
Hippocampus,  148 
Hlndbrain,  141 
Histology,  I,  16 
Holonephros,  310 
Holorhinal,  96 
Homocercal,  50 
Honey  comb,  227 
Hoofs,  27 

Hormones,  18,  353 
Horns,  10 1 
Humerus,  116 
Humors  of  eye,  203 
Hyoid  apparatus,  220 
Hyoid  arch,  63 
Hyoideus  nerve,  172 
Hyomandibular  bone,  73 

cartilage,  6^ 

nerve,  172 
Hyoplastron,  42 
Hyostylic,  73 
Hyparterial  bronchi,  262 
Hypaxial  muscles,  127 
Hypobranchial,  65 
Hypocentrum,  47 
Hypocone,  214 
Hypoconid,  214 
Hypogastric  artery,  276,  285 

plexus,  163 

vein,  290 
Hypoglossal  muscles,  128 

nerve,  177 
Hypoischium,  iii 
Hypomere,  13 
Hypopharyngeals,  80 
Hypophysial  duct,  191 
Hypophysis,  148 
Hypoplastron,  42 
Hypurals,  50 

Ichthyopterygia,  114 
Ileum,  228 
Ileo-caecal  valve,  228 
Ileo-colic  valve,  228 
Ileocostal  muscle,  131 
Iliac  artery,  288 

vein,  289 
Ilium,  109 

Incisive  foramina,  10 1 
Incisors,  213 
Incus,  74 
Inferior  jugular  vein,  278 

mesenteric  artery,  285 

oblique  muscle,  128 

turbinal,  100 
Infraclavicle,  106 
Infratemporal  fossa,  71 
Infundibulum,  148,  256 
Ingluvies,  223 
Inner  ear,  183 
Innominate  vein,  300 


Insertion  of  muscles,  129 
Insula  160 
Integument,  25 
Interbranchial  septum,  237 
Intercalare,  47 
Intercellular  substance,  21 
Intercentrum,  48 
Intercerebral  fissure,  148 
Interclavicle,  59 
Intercostal  arteries,  275,  286 

muscles,  130 
Interhyal,  80 

Intermaxillary  glands,  220 
Intermedium,  117 
Internal  iliac  artery,  288 

vein,  290 
Internal  secretion,  18 
Internasal  gland,  220 
Interoperculura,  77 
Interorbital  septum,  61 
Interparietal  bone,  68 
Interrenal  organs,  352 

veins,  291 
Interspinous  ligament,  48 
Interstitial  cells,  342 
Intertemporal  bones,  100 
Intestine,  227 
Intratarsal  joint,  118 
Intrinsic  muscles,  131 
Invagination,  9 
Inverted  eye,  200 
Involuntary  muscles,  20,  125 
Iris,  202 
Ischiadic  artery,  288 

vein,  290 
Ischio-pubic  fenestra,  109 
Ischio-pubis,  112 
Ischium,  109 
Island  of  Riel,  160 
Isthmus,  141 
Iter,  143 
Ivory,  209 

Jacobson's  commissure,  165,  171 

gland,  194 

organ,  190,  196 
Jaws,  63 
Jejunum,  228 
Jugal  bone,  70 
Jugular  ganglion,  176 

lymph  sac,  303 

vein,  279 

Kidney,  310 

development  of,  316 
Krausse's  corpuscle,  179 

Labial  cartilages,  65 

glands,  220,  221 
Labyrinth  of  ear,  185 

nasal,  192 
Lacrimal  bone,  69 

duct,  204 


INDEX. 


393 


Lacrimal  gland,  204 

Lacteals,  304 

Lacunae,  23 

Lagena,  184 

Lamina  terminalb,  141 

LarjTigeal  cartilages,  64 

Laryngeal  ventricle,  253 

Larynx,  250,  251 

Lateral  abdominal  vein,  289 

column  of  cord,  139 

comu,  139 

ethmoid,  67 

line  lobe,  145 

line  organs,  173,  179 

plate,  13 
Lateralis  nervous  system,  173 
Latissimus  dorsi,  132 
Legs,  116 
Lens  of  eye,  199 
Leptocardii,  2 
Leucocytes,  265 
Levator  muscles,  131 

scapulae,  132 
Leydig's  duct,  315 
Lids  of  eye,  203 
Ligament,  interspinous,  48 

of  ovar}',  338 

of  testis,  338 
Ligamentum  medium  pelvis,  in 

teres,  289 
Linea  alba,  127 
Lingual  glands,  221 
Lingualis  nerve,  171 
Lips,  208 
Liver,  231 
Longissimus  capitis  muscle,  13 1 

dorsi  muscle,  131 
Lophodont,  214 
Lophs,  214 

Lorenzini's  ampullae,  182 
Lower  jaw,  71 
Lumbar  arterj-,  2  86 

plexus,  163 

vertebrae,  49 
Luminous  organs,  28 
Lunatum,  117 
Lungs,  250,  255 
Lungs,  phylogeny  of,  262 
Lung  pipes,  259 
Lungless  salamanders,  299 
Lutein  cells,  320 
Lymph,  265 

glands,  302,  306 

hearts,  302,  304 

nodules,  306 

sacs,  303 
•stomata,  302 

vessels,  development  of,  302 
Lymphatic  system,  302 
Lymphocytes,  266 
Lyssa,  219 

Macula  lutea,  200 


Maculae  acusticae,  185 

Malar  bone,  70 

Male  ducts,  321 

Malleus,  74 

Malpighian  corpuscle,  309,  314 

Malpighian  layer,  25 

Mammals,  brain,  158 

circulation,  300 

dermal  skeleton,  41 

excretory  organs,  328 

foetal  envelopes,  348 

gill  pouches,  237,  244 

girdles  of,  108,  113 

glands  of,  35 

intestine,  230 

larynx,  254 

limbs,  119 

lungs,  262 

reproductive  organs,  335 

salivary  glands,  221 

skin  of,  33 

skull  of,  98 

stomach,  225 

teeth,  213 

thymus,  246 

thyreoid,  247 

tongue,  219 

vertebral  column  of,  53 
Mammary  glands,  36 
Mandibular  arch,  63 

arteries,  271 

nerve,  171 
Mantle  of  cerebrum,  141 
Manubrium,  56 

mallei,  74 
Manus,  116 
Manyplies,  227 
Marsupial  bones,  114 
Masseter  muscle,  133 
Mastoid  process,  100 
Matrix,  21 

Maxillaris  extemus  nerve,  172 
Maxillar)'  bone,  70 

nerve,  171 
Maxillo-turbinals,  196 
Meatus,  external  auditory,  187 
Meckelian  cartilage,  63,  71 
Mediastinum,  16,  122 
Medullary  cords,  321 

groove,  II 

plate,  II 

sheath,  19 
Medulla  oblongata,  142 
Meibomian  glands,  205 
Meissner's  corpuscle,  179 
Membrana  tectoria,  186 
Membrane  bones,  42,  65 

bones  of  skull,  68 
Membranous  lab}Tinth,  185 

skeleton,  37 
Meninges,  151 
Meninx  primitiva,  151 
Mento-Meckelian  bone,  71 


394 


INDEX. 


Merkel's  corpuscle,  179 

Mesencephalon,  142,  145 

Mesenchyme,  10,  16 

Mesenteric  arteries,  284 

Mesenteries,  14,  121 

Mesenteron,  205 

Mesethmoid,  67 

Mesobronchus,  259  • 

Mesocardia,  16,  122,  270 

Mesocolon,  122 

Mesoderm,  10 

Mesogaster,  15,  122 

Mesohepar,  15,  121 

Mesomere,  13 

Mesonephric  tubules,  313 
ducts,  313,  315 

Mesonephros,  310,  313 

Mesopterygium,  115 

Mesopterygoid,  80 

Mesorchium,  16,  122,  319 

Mesothelium,  10 

Mesorectum,  15,  122 

Mesovaria,  16,  122, 319 

Metacarpals,  117 

Metacarpus,  116 

Metacoecle,  123,  143 

Metacone,  214 

Metaconid,  214 

Metamerism,  13 

Metanephros,  310 
.Metapodium,  116 

Metapterygium,  115 

Metapterygoid,  80 
Metatarsale,  117 
Metatarsus,  116 

Metazoa,  i 

Metencephalon,  142 

Midbrain,  141,  145 

Middle  ear,  187 
plate,  13 
turbinal,  100 
Milk  dentition,  211 
glands,  36 
line,  36 
points,  36 
Minimus,  117 
Mitral  valve,  281 
Mixipterygium,  116,  343 
Molar  gland,  221 
Molars,  213 
Monimostylic,  88 
Monophyodont  dentition,  212 
Monorhinal,  191 
Monro,  foramen  of,  143 

sulcus  of,  141 
Morphology,  i 
Mossy  cells,  20 
Motor-nerve  root,  162 
Mouth,  208 
Miillerian  duct,  315 
Multangulum,  117 
Multicellular  glands,  18 
Muscle  plate,  13 
Muscles  of  appendages,  development  of,  131 


Muscles  of,  dermal,  134 

visceral,  132 
Muscular  system,  124 

tissue,  20 
Myelencephalon,  142 
Myocardium,  269 
Myocoele,  14,  121,  126 
Myocommata,  127 
Myoepicardial  mantle^  270 
Myofibrillae,  20 
Mylohyoid  muscle,  133 
Myosepta,  38,  127 
Myotomes,  14,  121,  127 

Nails,  27 

Nares,  external,  190 

internal,  193 
Naro-hypophysial  duct,  191 
Nasal  bones,  68 

capsules,  190 
Naso-palatal  canal,  197 
Naso-pharyngeal  duct,  194 
Naso-turbinals,  195 
Navel  cord,  351 
Naviculare,  117 

pedis,  117 
Nepopallium,  148 
Nephridia,  308 
Nephridial  arteries,  275,  285 
Nephrotomes,  14,  308,  311 
Nerve  cell,  19 
Nerve-end  apparatus,  178 
Nerve  of  Weber,  173,  189 
Nervous  system,  137 

central,  138 

development  of,  137 

tissue,  19 
Neural  arch,  45 

crest,  161 

folds,  II 

plate,  II 

spine,  46 
Neurapophysis,  45 
Neurenteric  canal,  12 
Neuroglia,  20,  139 
Neuromasts,  167 
Neuron,  19 
Neuropore,  12 
Nictitating  membrane,  203 
Non-elastic  tissue,  22 
Nose,  197 
Notochord,  12,  44 

sheath  of,  45 
Nuchal  flexure  of  brain,  143 
Nuclei  of  brain,  144 
Nucleus  dentatus,  145 

Oblique  muscles,  130 

of  eye,  128 
Obturator  foramen,  109 
Occipitalia,  66 
Occipital  bone,  68 
lobes,  159 


INDEX. 


395 


Occipital  vertebrae,  62 
Oculomotor  nerve,  170 
Odontoblasts,  39,  209 
Odontoid  process,  50 
(Esophago-cutaneus  duct,  239 
(Esophagus,  222 
Olecranon  process,  120 
Olfactory  bulb,  142,  167 

duct,  194 

lobe,  141 

nerve,  167 

organs,  189 

sac,  190 

tract,  142 
Oliva,  145 
Olivar}'  bodies,  145 
Omasum,  227 
Omentum,  15,  122 
Omostemum,  57 
Omphalomesaraic  artery,  276 

vein,  271 
Omphalomesenteric  artery,  276 

vein,  271 
Ontogeny,  i 
Operculare,  77 
Opercular  gill,  241 
Operculum,  77,  240 
Ophthalmic  nerve,  171 
Ophthalmicus  profundus  nerve,  171 

superficialis  nerve  of  seventh,  172 
of  fifth,  171 
Opisthocielous,  46 
Opisthotic,  67 
Optic  capsule,  202 

chiasma,  169 

cup,  198 

ganglion,  169 

lobes,  142,  169 

nerve,  169,  200 

pedicel,  203 

recess,  141 

stalk,  198 

thalami,  142,  146 

tract,  146 

vesicle,  198 
Oral  cavity,  208 

glands,  220 

hood,  208 

plate,  205 
Orbicular  muscles,  134 
Orbital  gland,  221 
Orbitosphenoid,  67 
Organ  of  Corti,  186 

of  Jacobson,  190,  196 
Origin  of  muscles,  129 
Oronasal  groove,  193 
Os  cloacae,  in 

en  ceinture,  86 

entoglossum,  97,  218 

pubis,  109 

transversum,  88 

uncinatum,  96 
Ossa  suprastemalia,  58 


Ossein,  23,  39 

Ossicula  auditus,  73,  187 

Ossification,  43 

Osteoblasts,  39 

Ostium  tubae  abdominale,  324 

Otic  bones,  67 

capsule,  60,  67 

ganglion,  171 
Otoliths,  185 
Ovarial  cords,  320 
Ovarian  artery,  286 
Ovaries,  308,  320 
Oviduct,  316,  321,  323 
Ovo testis,  346 
Ovum,  8 

Pacini's  corpuscle,  179 
Paired  appendages,  103 

fins,  114 
Palatal  glands,  220,  221 
Palatine  bones,  69 

nerve,  172 
Palatoquadrate  cartilage,  63 
Pallium,  141,  148 
Pancreas,  234 
Pancreatic  duct,  235 
Panniculus  camosus,  134 
Parabronchi,  259 
Parachordal  plates,  60 
Paracone,  214 
Paraconid,  214 
Paraglossae,  218 
Paraglossal,  97 
Paramastoid  process,  99 
Paraphysis,  146 
Parapophysis,  46 
Parasphenoid,  69 
Parietal  bones,  68 

eye,  147 

foramen,  68 

lobes,  159 

muscles,  125 

organ,  147 
Paroccipital  process,  99 
Parotic  process,  93 
Parotid  gland,  221 

of  Amphibia,  29 
Parovarial  canal,  325 
Parovarium,  341 
PateUa,  118,  120 
Paunch,  227 
Pearl  organs,  27 
Pecten  of  eye,  204 
Pectineal  process,  no,  113 
Pectineus  muscle,  132 
Pectoral  girdle,  105 
Pectoralis  muscle,  132 
Pedimcles  of  cerebellum,  150 
Pelvic  girdle,  109 

plexus,  163 
Pelvis,  109 

of  kidney,  330 
Penis,  345 


396 


INDEX. 


Pepsin,  224 

Pericardial  cavity,  14,  123 

fluid,  269 
Pericardio-peritoneal  canals,  123,  271 
Pericardium,  124,  269 
Perichondrium,  39 
Periderm,  25 
Peridural  space,  152 
Perilymph,  186 

duct,  186 
Perimeningeal  space,  151 
Perimysium,  21,  128 
Perineurium,  20 
Periosteum,  24,  39 
Peripheral  nervous  system,  161 
Peristalsis,  207,  227 
Peritoneal  canals,  124 

cavity,  14,  123 
Peritoneum,  124 
Permanent  dentition,  211 
Peroneal  arter}%  288 
Perpendicular  plate  of  ethmoid,  100 
Pes,  116 
Pessulus,  255 
Petrosal  bones,  67 

ganglion,  175 
Petrotympanic  fissure,  74 
Phasochrome  cells,  352 
Phalanges,  116 
Pharyngeal  bones,  80 

derivatives,  245 

plate,  205 

tonsils,  247 
Pharyngobranchial,  65 
Pharynx,  207,  222,  236 
Phosphorescent  organs,  28 
Photophores,  28 
Phyllospondylous,  47 
Physiology,  i 
Physoclistous,  248 
Physostomous,  248 
Pia  mater,  152 
Pigment  cells,  22,  26 

layer  of  eye,  198 
Pigmented  epithelium  of  eye   20  r 
Pillar  cells,  186 
Pinealis,  147 
Pisiforme,  117 
Pituitary  body,  148 
Placenta,  318,  351 

vitelline,  348 
Placoid  scale,  40 
Plantigrade,  120 
Plasma,  blood,  265 
Plastron,  41 
Platybasic  skull,  61 
Platysma  myoides,  134 
Pleura,  124 
Pleural  cavities,  123 

rib,  53 
Pleurapophysis,  46 
Pleurocentrum,  47 
Pleurodont.  88 


Pleurodont  dentition,  2 13 
Plexus,  chorioid,  144 
Plexuses,  163 
Plica  fimbriata,  219 

semilunaris,  203 
Plumae,  31 
Plumulae,  31 
Pneumatic  bones,  96 

duct,  248 
Pneumatocyst,  247 
Pneumogastric  nerve,  176 
Podium,  116 
Poison  glands,  27,  221 
Pollex,  117 
Polymastism,  37 
Polyphyodont  denition,  211 
Pons  (Varolii),  150 
Pontal  flexure  of  brain,  144 
Popliteal  artery,  288 
Pori  abdominales,  124,  322 
Portal  circulation,  277 

heart,  294 

vein,  277 
Postbranchial  bodies,  246 
Postcardinal  vein,  279 
Postcava,  290 
Postclavicle,  106 
Posterior  chamber  of  eye,  203 

column  of  cord,  139 

cornua,  139 

fissure  of  cord,  139 

horns  of  cord,  139 

lymph  sac,  303 
Postfrontal  bone,  69 
Posthepatic  digestive  tract,  206 
Postminimus,  118 
Postorbital  bone,  69 
Postotic  nerves,  175 
Postparietal  bone,  69 
Postpermanent  dentition,  212 
Postpubic  process,  112 
Post-temporal  bone,  106 

fossa,  71 
Post-trematic  nerves,  175 
Postzygapophysis,  46 
Precava,  300 
Predentary  bone,  88 
Prefrontal  bones,  69 
Prefrontals  of  birds,  96 
Prehallux,  118 

Prehepatic  digestive  tract,  206 
Prelacteal  dentition,  212 
Premaxillary  bone,  70 
Premolars,  213 
Prenasal  bone,  100 
Preoperculum,  77 
Prepollex,  118 
Prepubic  process,  iii 
Presphenoid,  67 
Presternalia,  109 
Pretrematic  nerves,  175 
Prevertebral  ganglia,  163 
Prevomer,  69 


INDEX. 


397 


Prezygapophysis,  46 
Primilive  groove,  to 

ova,  319 

streak,  10 
Primordial  ova,  ^nj 
Process,  articular,  46 

transverse,  46 
Proccelous,  46 
Procoracoid,  107 
Proctodeum,  13,  ?o6 
Pronephric  duct,  312 

tubules,  311 
Pronephros,  310 
Prootic,  67 
Propterygium,  115 
Prosencephalon,  141 
Prostate  glands,  342 
Prostemum,  58 

Proterandric  hermaphroditism,  346 
Proteroglypha,  213 
Protocone,  214 
Protoconid,  214 
Pro  to  vertebrae,  14 
Protractor  muscles,  131 
Proventriculus,  225 
Psalterium,  227 
Pseudobranch,  241 
Pseudoconch,  194 
Pterotic,  67 
Pterygoid  bones,  69,  80 

muscle,  133 
Pterygoquadrate,  63,  69 
Pterylae,  32 
Ptj-alin,  220 

Pubofemoralis  muscle,  132 
Pubis,  109 
Pulmonar>-  arteries,  283 

circulation,  282 

veins,  292 
Pulmones,  236 
Pulp  of  tooth,  210 
Pupil,  202 
Pygostyle,  53 
Pyloric  caeca,  227 

gland,  224 
Pylorus,  223 
P}Tamidalis  musde,  130 
Pyramidal  tracts,  150 
PjTiform  lobes,  159 
Pyriformis  muscle,  132 

Quadrate  bone,  69,  74 
Quadratogugal  bone,  70 
Quadritubercular,  214 

Radial  arter)-,  288 

cells,  200 
Radiale,  117 
Radialia,  103 
Radius,  116 
Radix  aortae,  273 
Rami  communicantes,  163 
Ramus  dorsalis,  162 


Ramus  intestinalis,  162 

of  tenth  nerve,  176 

lateralis  accessorius,  173 
of  tenth  nerve,  176 

ventralis,  162 

visceralis,  162 
Rathke's  pocket,  148,  206 
Rectal  gland,  228 
Rectxun,  228 
Rectus  abdominis  muscle,  130 

capitis  muscles,  131 

muscles,  128,  130 
of  eye,  128 
Red  spots,  249 
Reissner's  fibres,  151 
Renal  artery,  286,  300 

corpuscle,  309 

portal  system,  280 
Rennet,  227 
Respiration,  accessor)'  structures,  263 

mechanism  of,  263 
Respiratory  circulation,  282 

duct,  194 

organs,  235 
Reproductive  ducts,  321 

organs,  308,  319 
Reptiles,  see  also  Sauropsida,  Amniotes. 

aortic  arches,  283 

brain,  157 

circulation,  299 

copulatory  organs,  344 

dermal  skeleton,  41 

gill  pouches,  244 

girdles,  107,  iii 

glands,  30 

intestine,  229 

larynx,  251 

limbs,  118 

limgs,  258 

scales,  30 

skull,  87 

thymus,  246 

thyreoid,  247 

tongue,  217 

vertebral  column,  52 
Rete  mirabile,  249,  267 
Reticulum,  227 
Retina,  199 
Retinal  arter>',  201 

ganglion,  200 

layer  of  eye,  198 

vein,  201 
Retractor  bulbi,  128,  203 

muscles,  131 
Retrolingual  gland,  221 
Revehent  vein,  291 
Rhachitomous,  48 
Rhinencephalon,  141 
Ribs,  53 

abdominal,  41 
Rods  of  eye,  199 
Roof  plate,  138 
Roots  of  spinal  nerves   161 


398 


INDEX. 


Rostral  bone,  88 

cartilage,  76,  85 
Rostrum  sphenoidale,  96 
Rotator  muscles,  132 
Rumen,  227 

Saccule- ventricular  canal,  184 
Sacculus  endolymphaticus,  183 

of  ear,  184 
Saccus  vasculosus,  148 
Sacral  artery,  286 

plexus,  163 

vertebrae,  49 
Sacrum,  49 
Salivary  glands,  220 
Santorini's  duct,  235 
Sarcolemma,  21 
Saurognathous,  97 
Sauropsida,  eyes,  204 

excretory  organs,  327 

foetal  envelopes,  348 

reproductive  organs,  334 

teeth,  212 
Savi's  ampullae,  182 
Scala  media,  185 
Scalae  of  ear,  186 
Scalene  muscles,  130 
Scales,  26,  39 

ctenoid,  40 

cycloid,  40 

development  of,  39 

ganoid,  40 

of  mammals,  34 

placoid,  40 
Scaphoid,  117 
Scapular  region,  105 
Schizocoele,  10 
Schizorhinal,  96 
Sciatic  artery,  288 

vein,  290 
Sclera,  62,  202 
Scleroblasts,  39 
Sclerotic  bones,  67,  203 

coat,  62 
Sclerotomes,  14 
Scrotum,  321,  338 
Secodont,  214 
Seessel's  pocket,  206 
Segmentation  cavity,  8 

of  egg,  8 
Selenodont,  214 
Sella  turcica,  61 
Semicircular  canals,  184 
Semilunar  fold,  203 

ganglion,  171 
Seminal  vesicles,  342 
Seminiferous  tubule,  321 
Sense  hillocks,  167 
Sensory  epithelium,  17 

nerve  root,  162 

organs,  177 
Septum,  interorbital,  61 

of  cerebrum,  148 


Septum  pellucidum,  151 

transversum,  123,  271 
Serosa,  350 

Serous  coat  of  digestive  tract,  207 
Serratus  anterior  muscle,  132 
Sertoli's  cells,  321 
Serum,  265 
Sesamoid  bones,  129 
Sheath  of  Schwann,  20 
Shoulder  girdle,  105 
Sex,  determination  of,  347 
Sexual  cords,  320 

organs,  308 
Sight,  organs  of,  198 
Sinus,  cervical,  244 

frontal,  197 

impar,  250 

maxillary,  197 

of  Morgagni,  253 

sphenoidal,  197 

terminalis,  277 

urogenitalis,  322 

venosus,  272 
Sinusoids,  267 
Sixth  sense,  182 
Skeletal  labyrinth,  186 
Skeleton,  37 

appendicular,  102 

axial,  43 

dermal,  38 

membranous,  37 

visceral,  63 
Skin,  25 

of  mammals,  ^^ 
Skull,  59 

development  of,  59 

table  of  bones  of,  72 
Small  omentum,  122 
Smell,  organs  of,  r89 
Smooth  muscles,  20    124 
Soft  commissure,  146 
Solar  plexus,  163 
Solenoglypha,  213 
Somatic  layer,  10 

motor  nerves,  165 

nerves,  162 

wall,  121 
Somatopleure,  15,  121 
Spermatic  artery,  286 
Spermatozoon,  8 
Sphenethmoid,  86 
Sphenoid  bone,  68 
Sphenoidalia,  67 
Sphenoidal  fissure,  67 

turbinal,  100 
Sphenopalatine  ganglion,  165,  171 
Sphenotic,  67 

ganglion,  165 
Sphincter  muscles,  129 

pupillae,  202 
Spina  scapulae,  109 
Spinal  artery,  287 

cord,  138 


INDEX. 


399 


Spinal  muscles,  131 

nerves,  161 
Spiracle,  238 
Spiral  valve,  228 
Splanchnic  layer,  10 

wall,  121 
Splanchnoccele,  121 
Splanchnopleure,  15,  121 
Spleen,  307 
Splenial  bone,  71 
Splenic  artery,  284 
Squamosal  bone,  70 
Squamous  epithelium,  18 
Squatina,  genus  of  sharks. 
Stapes,  73,  186 
Steapsin,  234 
Stenon's  duct,  221 
Stenson's  gland,  197 
Sternal  rib,  54 
Stemebrae,  57 

Sternocleidomastoid  muscle,  130 
Sternohyoid  muscle,  130 
Sternum,  56 

abdominal,  57 
Stomach,  223 

Stomata  of  lymph  system,  302 
Stomodeum,  12,  205 
Stratified  epithelium,  18 
Stratum  comeum,  25 

germinativum,  25 
Streptostylic,  87 
Striped  muscles,  20,  125 
Styloid  process,  100 
Subarachnoid  space,  152 
Subcardinal  veins,  279 
Subclavian  artery,  288 

vein,  289 
Subcutis,  26 
Subdural  space,  152 
Sub  intestinal  vein,  276 
Sublingua,  219 
Sublingual  gland,  221 
Submaxillary  ganglion,  171 

gland,  221 
Suboperculum,  77 
Subspinal  muscles,  133 
Subunguis,  27 
Sulci,  160 

Sulcus  of  Monro,  141 
Superficial  petrosal  nerve,  173 
Superior  intercostal  vein,  302 

jugular  vein,  279 

mesenteric  artery,  284 

oblique  muscle,  128 

turbinal,  100 
Supracleithra,  106 
Supracondylar  foramen,  120 
Supraethmoid,  80 
Suprangulare,  71 
Supraoccipital,  67 
Supraorbital  bones,  69 
Suprapericardial  bodies,  246 
Suprarenal  organs,  352 


Suprascapula,  105,  107 
Suprasternalia,  58 
Supratemporal  bone,  69,  106 

fossa,  71 
Suspensor,  63,  69,  73 
Sutures,  38 
Sweetbreads,  246 
Swim  bladder,  247 
Sylvian  fissure,  159 
Sympathetic  ganglia,  165 

system,  163 

trunk,  163 
Symplectic,  73,  80 
Synarthrosis,  38 
Synotic  tectum,  61 
Synovial  membrane,  38 
Synsacrum,  53 
Syrinx,  254 

Systemic  circulation,  282 
Systole,  272 

Tabulare,  69 
Tactile  corpuscles,  179 
Taenia  marginalis,  98 
Tails  of  fishes,  50 
Talon,  214 
Talus/  117 

Tapetum  lucidum,  202 
Tarsale,  117 
Tarsal  glands,  205 
Tarso-meta  tarsus,  119 
Tarsus,  116 
Taste  buds,  189 

organs  of,  189 
Tear  gland,  204 
Tectorial  membrane,  186 
Teeth,  208 

development  of,  209 

epidermal,  215 

phylogeny,  215 
Tegmen  cranii,  61 
Tela  subjunctiva,  26 
Telencephalon,    141,  148 

brain,  153 

breathing  valves^  241 

excretory  organs,  327 

girdles,  105 

skull,  77 

reproductive  organs,  332 
Temnospondylous,  48 
Temporal  fossa,  7 1 

lobes,  159 
Temporalis  muscle,  133 
Tenaculum,  203 
Tendons,  129 
Tentorium,  152 
Terminalis  nerve,  169 
Testes,  308,  320 

descent  of,  338 
Thalamencephalon,  142 
Thalamic  nerve,  169 
Thalamus,  142,  146 
Thecodont,  88,  213 


400 


INDEX. 


Thoracic  aorta,  284 

duct,  303 

vertebrae,  49 
Thread  cells,  29 
Thymus  glands,  245 
Thyreoid  cartilage,  252 

gland,  246 
Thyrohyals,  102 
Tibia,  116 
Tibiale,  117 
Tibial  artery,  288 
Tibio-tarsus,  119 
Tissue,  16 
Tongue,  217 
Tonsils,  247,  307 
Trabecula  cranii,  61 

communis,  61 
Trachea,  250,  254 
Tractus  oKactorius,  142,  167 

solitarius,  150 
Transverse  bone,  88 

muscles,  130 

process,  46,  55 
Transverso-spinal  muscles,  131 
Trapezium,  117 
Trapezius  muscle,  132 
Trapezoides,  117 
Triconodont,  214 
Tricuspid  valve,  281 
Trigeminal  nerve,  170 
Triquetrum,  117 
Tritubercular,  214 
Trochanter,  120 
Trochlearis  nerve,  170 
Tropibasic  skull,  61 
Truncus  arteriosus,  272 

transversus,  271 
Trypsin,  234 
Tuber  acusticum,  145 
Tubular  glands,  18 
Tubercular  head  of  rib,  54 
Tuberculum  impar,  217 
Tunica  albuginea,  341 

serosa,  121 

vasculosa  of  eye,  201 
Tunicata,  2 

Turbinal  bones,  67,  100,  195 
Turtles,  armor  of,  41 
'Twixt-brain,  142,  148 
Tympanic  annulus,  82 

bone,  100 

membrane,  187 
Tympanum  of  ear,  187 

of  syrinx,  255 

Ulna,  116 

Ulnare,  117 
Ulnar  artery,  288 

lymph  duct,  303 
Umbilical  artery,  285 

cord,  351 

veins,  278 

vesicle,  348 


Umbilicus  of  feather,  31 
Unguis,  27 
Unguligrade,  120 
Uncinate  bone,  97 
Unicellular  glands,  18 
Uniserial  fin,  115 
Upper  jaw,  70 
Ureter,  318 
Urethra,  319,  331 
Urinary  bladder,  318 

organs,  307 
Urocyst,  318 
Urogenital  sinus,  322 

system,  307 
Urohyal,  80,  97,  218 
Uropygial  glands,  30 
Urostyle,  52 
Uterus,  338 

masculinus,  342 
Utriculus,  183 
Uvea,  202 

Vagina,  338 
Vagus  nerve,  175 
Valve,  ileocascal,  228 

ileo-colic,  228 

of  Vieussens,  145 

spiral,  228 
Vas  eflferens,  321 
Vasa  deferentia,  321 
Vascular  cells,  270 
Vater's  corpuscle,  179 
Veins,  266,  276,  289 

abdominal,  289 

advehent,  291 

allantoic,  350 

anterior  abdominal,  289 
cardinal,  279 

axillary,  290 

azygos,  302 

brachial,  290 

branchial,  274 

caudal,  276 

central  retinal,  201 

cephalic,  290 

common  iliac,  289 

cutaneus  magnus,  290 

epigastric,  289 

femoral,  290 

hemiazygos,  302 

hepatic,  277 

hypogastric,  290 

iliac,  289 

inferior  jugular,  278 

innominate,  300 

internal  iliac,  290 

interrenal,  291 

ischiadic,  290 

jugular,  279 

lateral  abdominal,  289 

omphalomesaraic,  271 

omphalomesenteric,  271 

portal,  277 


i 


i 


INDEX. 


401 


Veins,  postcardinal,  279 

pulmonary,  292 

revehent,  291 

sciatic,  290 

subcardinal,  279 

subclavian,  289 

sub  intestinal,  276 

superior  intercostal,  302 
jugular,  279 

umbilical,  278 

vitelline,  277 

vertebral,  292 
Velum  medullare  anterius,  145 

transversum,  146 
Vena  cava,  anterior  300 

inferior,  290 
Ventral  aorta,  273 

nerve  root,  161 
Ventricles,  cornua  of,  160 

fifth,  151 

lar>Tigeal,  253 

of  brain,  12,  142 

of  heart,  272 

of  lungs,  259 
\'ermis,  145 

Vertebra,  development  of,  48 
Vertebrae,  45 

occipital,  62 
Vertebral  artery,  287 

column,  44 

rib,  54 

vein,  292 
Vertebraterial  canal,  54 
Vertebrata,  2 
Vesical  arteries,  285 
Vestibular  nerve,  174 
Vestibule  of  mouth,  208 

of  nose,  194 
Vestibulum,  183 
Mdian  nerve,  165 
Vieussens,  valve  of,  145 
Villi,  227 
Visceral  arches,  63 

clefts,  236 


Visceral  motor  nerves,  165 

muscles,  132 

nerves,  163 

pouches,  236 

sensory  nerves,  167 

skeleton,  63 
Vitelline  veins,  277 
Vitreous  body,  200 
Vocal  cords,  251,  253 

sacs,  253 
Volimtarj'  muscles,  20,  125 
Vomer,  69 
Vomero-nasal  organ,  196 

Weberian  apparatus,  54,  250 
Weber's  nerve,  189 
Whalebone,  216 
WTiariion's  duct,  221 
\Miite  matter,  20 

of  cord,  139 

tissue,  22 
Willis,  circle  of,  287 
Winslow's  foramen,  122 
Wirsung's  duct,  235 
Wishbone,  108 
Wolffian  body,  310,  313 

duct,  315 

ridge,  311 

Xiphioid  process,  56 
Xiphistemum,  57 

Yellow  spot,  200 
Yolk,  8 

sac,  206,  277,  348 
Ypsiloid  cartilage,  no 

2k)nes  of  nervous  system,  1 4  r 
Zonula  ciliaris,  202 

Ziimii,  202 
Zygantra,  52 
Zygapophysis,  46 
Zygomatic  bone,  70 
Zygosphenes,  52 


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Biology,  James  Milliken  University,  Decatur,  Illinois.  Second  Edition, 
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Elementary  Zoology.  A  Text-book  for  Secondary  Educational 
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GREEN.  Vegetable  Physiology,  An  Introduction  to.  By  J.  Reynolds 
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JOHNSTON.  Nervous  System  of  Vertebrates.  By  John  Black 
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VINAL.    A  Guide  for  Laboratory  and  Field  Studies  in  Botany.     By 

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